Adding VM based locks to ovf modify functions.

Caveat Emptor:
Avoids concurrent runs, but not serializes.
Simultaneous updates or removes of ovf files (like removing many
disks from the same VM) may result in inconsistent ovf's.

Bug-url: https://bugzilla.redhat.com/show_bug.cgi?id=878894

Change-Id: I57d9f87a34e1d5659115b09530493824fcda94de
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/9456
Reviewed-by: Ayal Baron <abaron@redhat.com>