Fix duplicate port mirrorring when reattaching vm

When there is a VM running with port mirrorring defined, the fact
that a vdsm restart re-attaches by "recreating" the vdsm vm object
makes it run setPortMirroring again. This caused every vdsm restart
to add a mirrorring duplication.

Change-Id: I63deb5e703060f1891779d6697eaf17cd06ce8d3
Bug-Url: https://bugzilla.redhat.com/1001704
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19534
Reviewed-by: Dan Kenigsberg <danken@redhat.com>