virttest.utils_test.libvirt: update disk source of the VM

To update disk source path of the VM according to specified direcotry, such as a NFS sharing directory.

Signed-off-by: Alex Jia <ajia@redhat.com>
Maybe you should check other disks on this vm too.
Because migration is still not working for other non-shared disks.
Or you can remove all other irrelative disks by another parameter.