hw/virtio: Introduce virtio_bh_new_guarded() helper

hw/virtio: Introduce virtio_bh_new_guarded() helper

Introduce virtio_bh_new_guarded(), similar to qemu_bh_new_guarded()
but using the transport memory guard, instead of the device one
(there can only be one virtio device per virtio bus).

Inspired-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20240409105537.18308-2-philmd@linaro.org>
(cherry picked from commit ec0504b989ca61e03636384d3602b7bf07ffe4da)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>