The tap backend is already using qemu-bridge-helper to attach tap
interface to a bridge but (unlike the bridge backend) it always uses
the default bridge name - br0.

This adds a "br" property support to the tap backend.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>