bug: file transfers need twice the disk space

[mkunz7]

Describe your environment

1. minimega version
   latest
2. Linux distro/version
   ubuntu 18.04.3
3. Go compiler version
   1.12.13
4. VM types
   all

Describe the bug
The file API doesn't throw an error when you run out of disk space.

If you send a 50gB file, you need to have at least 100gB's of free space.

To Reproduce
Start Minimega on 2 nodes
Create a large file (10gB) and put it in the file path on node1

On node2 only have 15 gB remaining. Mounting the file path in a ram drive helps.

From node2

file get mylargefile.qcow2

The file downloads, then truncates to 5gB without error to CLI. If you have multiple nodes downloading the file with mesh send all file get mylargefile.qcow2 once one node finishes all the qcows get corrupted before they finish downloading.

Expected behavior
Error on running out of disk space. Why is this a copy and not move operation?