hsm: fix sloppy rebase of one shot prepare

A sloppy rebase of c072945 "One shot prepare" removed the changes
introduced by cef2d5b "vm: extend shared property to support locking".

Change-Id: I8f0de78bb9865c93dc40f51c1fcc9b6b29cef516
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19049
Reviewed-by: Ayal Baron <abaron@redhat.com>