utils_test.libvirt: Add snapshot attribute for setting vm disk

In domain disk xml, the snapshot attribute accept 'no', 'internal'
and 'external' value, which used to allow or deny doing certain type
of snapshot on the disk.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1932 from waynesun09/disk_snapshot_attr

utils_test.libvirt: Add snapshot atrribute for setting vm disk