Fix bridge name selection for vswitch mode in scripts/setup-vif-rules

-        bridge_name = "xenbr%s" % devid
  +        bridge_name = get_bridge_name_vswitch(vif_name)

  Devid is device number for domU (f.e. vif1.15; 15 - devid) and is definitely NOT a
  xenbr number (xenbr0, xenbr1, etc).

Signed-off-by: George Shuklin <george.shuklin@gmail.com>