virtio-blk: use a helper to handle request queuing errors

Define a new helper function, virtblk_fail_to_queue(), to
clean up the error handling code in virtio_queue_rq().

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Message-Id: <20221016034127.330942-2-dmitry.fomichev@wdc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>