snapshot_disk: Fix error cases for internal diskonly and external revert

Internal disk only snapshot will fail since commit d2e668e in libvirt
1.2.5, and external revert will fail since d410e6f in libvirt 1.2.3.

Following bugs is tracking for this:
https://bugzilla.redhat.com/show_bug.cgi?id=1071264
https://bugzilla.redhat.com/show_bug.cgi?id=1083345

Signed-off-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Wayne Sun <gsun@redhat.com>