virtio-blk: drop unused virtio_blk_handle_vq() return value

virtio-blk: drop unused virtio_blk_handle_vq() return value

The return value of virtio_blk_handle_vq() is no longer used. Get rid of
it. This is a step towards unifying the dataplane and non-dataplane
virtqueue handler functions.

Prepare virtio_blk_handle_output() to be used by both dataplane and
non-dataplane by making the condition for starting ioeventfd more
specific. This way it won't trigger when dataplane has already been
started.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20211207132336.36627-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>