When invoking qemu-bridge-helper in 'net_bridge_run_helper',
instead of using fixed sized buffers, use dynamically allocated
ones initialised and returned by g_strdup_printf().

Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>