Add new disk commands

[jacdavi]

• Adds commands for manipulating disks (wrappers for qemu-img)
  • disk <rebase,> <image> [backing file]
  • disk <set-backing,> <image> [backing file] (rebase with unsafe flag)
  • disk resize <image> <size>
  • disk commit <image>
• Updates disk info with recursive option which prints entire backing chain
• Updates disk info with new column for "in use"
  • This is tracked by checking if qemu has a lock on the disk file. This will mark backing images of booted images as in use
• Change snapshot to use a backing file path relative to the base image rather than an absolute path
  • Change disk info output for backingfile to show relative path if backing file is relative
• Minor code restructure for readability
• Parse qemu-img info as json rather than regex on a plain string
• Format disk size as human readable IEC format