net tests: rename temporary variable

vdsm_net is too confusing with self.vdsm_net.

Change-Id: I7f7bddddc32a0014d83a3eeb60230fab0b19a26b
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35910
Tested-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-by: Ido Barkan <ibarkan@redhat.com>