vm: prevent faulty volume extensions

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=998443
Change-Id: If7e85c50f375ed7ed0297330b8cd83e38e0986c7
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21150
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21383
Reviewed-by: Dan Kenigsberg <danken@redhat.com>