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>
OK, I got your point.
When you can make sure the vm you provide is prepared for migration tests, It's good enough.
I have this question because if we provide a vm with some disks(exist maybe) which test is not clear, these disks except primary disk are out of control.