vm: snapshot transient disk check should be per disk

Live snapshot is allowed if all the disks in the snapshot request are
not transient.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1065886
Change-Id: I3775e2218186e56e99740c6f91c8c98484529892
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24867
Reviewed-by: Daniel Erez <derez@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>