virtio-blk: Fix rollback path in virtio_blk_data_plane_start()

virtio-blk: Fix rollback path in virtio_blk_data_plane_start()

When dataplane multiqueue support was added in QEMU 2.7, the path
that would rollback guest notifiers assignment in case of error
simply got dropped.

Later on, when Error was added to blk_set_aio_context() in QEMU 4.1,
another error path was introduced, but it ommits to rollback both
host and guest notifiers.

It seems cleaner to fix the rollback path in one go. The patch is
simple enough that it can be adjusted if backported to a pre-4.1
QEMU.

Fixes: 51b04ac5c6a6 ("virtio-blk: dataplane multiqueue support")
Cc: stefanha@redhat.com
Fixes: 97896a4887a0 ("block: Add Error to blk_set_aio_context()")
Cc: kwolf@redhat.com
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210407143501.244343-2-groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>