virtio-scsi: avoid dangling host notifier in ->ioeventfd_stop()

virtio-scsi: avoid dangling host notifier in ->ioeventfd_stop()

virtio_scsi_dataplane_stop() calls blk_drain_all(), which invokes
->drained_begin()/->drained_end() after we've already detached the host
notifier. virtio_scsi_drained_end() currently attaches the host notifier
again and leaves it dangling after dataplane has stopped.

This results in the following assertion failure because
virtio_scsi_defer_to_dataplane() is called from the IOThread instead of
the main loop thread:

  qemu-system-x86_64: ../softmmu/memory.c:1111: memory_region_transaction_commit: Assertion `qemu_mutex_iothread_locked()' failed.

Buglink: https://gitlab.com/qemu-project/qemu/-/issues/1680
Reported-by: Jean-Louis Dupond <jean-louis@dupond.be>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230611193924.2444914-1-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>