vm: shared attribute backward compatibility

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1011608
Change-Id: I4d377635b0687baccc69b203cb3fbe8dbf573169
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19509
Reviewed-by: Dan Kenigsberg <danken@redhat.com>