engine: Fix NPE in UpdateVmDiskCommand

The patch http://gerrit.ovirt.org/#/c/19446/ introduces a change which
set quota_id as NULL instead of Guid.Empty.

It causes NullPointerException in UpdateVmDiskCommand.

Change-Id: If689b77cfe4b4a466b70cd9194e99f0eeafcca09
Bug-Url: https://bugzilla.redhat.com/1015872
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>