tc: set/unset promisc when it's really needed

When setPortMirroring is called multiple times for the same network (in
case that more than one vms on the same host would monitor the same
logical network), we don't need to set the device's promisc flag to on
more than once.

Change-Id: I7d38e2f13a059ea916892ab8247509e2e0b612e2
Signed-off-by: huntxu <mhuntxu@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/12307
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>