Don't offer non-vector layer providers as options for virtual layer source layers

[nyalldawson]

This isn't supported, and results in a hang. Also cleanup and fix some more issues in the virtual layer provider.
Fixes #48908

[nirvn]

Completely cosmetic here: you could get rid of the empty line if you feel like waiting for another round of CIs to turn green again ;)

I've seen it elsewhere too (mesh provider for e.g.). I don't know if we have a convention on this or not. No problem if you leave those in.

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 87232a8234... Add QgsProviderMetadata::supportedLayerTypes() so that we can
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging python/core/auto_generated/providers/qgsprovidermetadata.sip.in
CONFLICT (modify/delete): src/core/providers/copc/qgscopcprovider.cpp deleted in HEAD and modified in 87232a8234 (Add QgsProviderMetadata::supportedLayerTypes() so that we can).  Version 87232a8234 (Add QgsProviderMetadata::supportedLayerTypes() so that we can) of src/core/providers/copc/qgscopcprovider.cpp left in tree.
CONFLICT (modify/delete): src/core/providers/copc/qgscopcprovider.h deleted in HEAD and modified in 87232a8234 (Add QgsProviderMetadata::supportedLayerTypes() so that we can).  Version 87232a8234 (Add QgsProviderMetadata::supportedLayerTypes() so that we can) of src/core/providers/copc/qgscopcprovider.h left in tree.
Auto-merging src/core/providers/ept/qgseptprovider.cpp
Auto-merging src/core/providers/ept/qgseptprovider.h
Auto-merging src/core/providers/gdal/qgsgdalprovider.cpp
Auto-merging src/core/providers/memory/qgsmemoryprovider.cpp
Auto-merging src/core/providers/ogr/qgsogrprovidermetadata.cpp
Auto-merging src/core/providers/ogr/qgsogrprovidermetadata.h
Auto-merging src/core/providers/qgsprovidermetadata.cpp
Auto-merging src/core/providers/qgsprovidermetadata.h
Auto-merging src/core/providers/qgsproviderregistry.cpp
Auto-merging src/core/vectortile/qgsvectortileprovidermetadata.cpp
Auto-merging src/providers/arcgisrest/qgsafsprovider.cpp
Auto-merging src/providers/arcgisrest/qgsafsprovider.h
Auto-merging src/providers/arcgisrest/qgsamsprovider.cpp
Auto-merging src/providers/arcgisrest/qgsamsprovider.h
Auto-merging src/providers/delimitedtext/qgsdelimitedtextprovider.cpp
Auto-merging src/providers/delimitedtext/qgsdelimitedtextprovider.h
Auto-merging src/providers/hana/qgshanaprovider.cpp
Auto-merging src/providers/hana/qgshanaprovider.h
Auto-merging src/providers/mssql/qgsmssqlprovider.cpp
CONFLICT (content): Merge conflict in src/providers/mssql/qgsmssqlprovider.cpp
Auto-merging src/providers/mssql/qgsmssqlprovider.h
Auto-merging src/providers/oracle/qgsoracleprovider.cpp
Auto-merging src/providers/oracle/qgsoracleprovider.h
Auto-merging src/providers/pdal/qgspdalprovider.cpp
Auto-merging src/providers/pdal/qgspdalprovider.h
Auto-merging src/providers/postgres/qgspostgresprovider.cpp
Auto-merging src/providers/postgres/qgspostgresprovider.h
Auto-merging src/providers/postgres/raster/qgspostgresrasterprovider.cpp
Auto-merging src/providers/spatialite/qgsspatialiteprovider.cpp
Auto-merging src/providers/spatialite/qgsspatialiteprovider.h
Auto-merging src/providers/wcs/qgswcsprovider.cpp
CONFLICT (content): Merge conflict in src/providers/wcs/qgswcsprovider.cpp
Auto-merging src/providers/wcs/qgswcsprovider.h
CONFLICT (content): Merge conflict in src/providers/wcs/qgswcsprovider.h
Auto-merging src/providers/wfs/qgsoapifprovider.cpp
Auto-merging src/providers/wfs/qgsoapifprovider.h
Auto-merging src/providers/wfs/qgswfsprovider.cpp
Auto-merging src/providers/wfs/qgswfsprovider.h
Auto-merging src/providers/wms/qgswmsprovider.cpp
Auto-merging src/providers/wms/qgswmsprovider.h

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-48988-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 87232a823451a174547c83d299457f47d18b80af,c8347e34120ffa3cc88c2653b083b269500775e9,8e58188776436e40dbb8343586e63ae026883790,637852022040e46e17435257ebd126a77cb8284d,b1db5d91990d8fcaf905a947878895cb30cb5e75,fe4d499d8dd765df5436d0c1fb0fd1a70fa7f848,2e9f754a5b1a3c94d34e1fcee02103e3a5c049a5
# Push it to GitHub
git push --set-upstream origin backport-48988-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-48988-to-queued_ltr_backports.