vm: pre-validate disks for snapshot

This patch divides the snapshot disks validation from the path
preparation in order to limit the rollback scenarios.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1065886
Change-Id: I4fb507c2f7268bb688af5fd187dcd033d0a068a2
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20029
Reviewed-by: Dan Kenigsberg <danken@redhat.com>