Root.disk migration - warn about unusable /home, /usr

[bcbc]

When using the --root-disk= option, the script (checksource.sh) requires that additional virtual disks are in the /hosts/ubuntu/disks folder. If they are, but they're not available or are already mounted, there is an error message. If not, there is no message.

It should at least warn that it cannot use the /home or /usr virtual disk because it's non-standard. It could also be a little more flexible e.g. if the virtual disk is xxx.disk, who cares whether it's in /host/ubuntu/disks as long as it's in the same location as the root.disk.