virtio-scsi: fix various bad behavior on aborted requests

Even though the virtio-scsi spec guarantees that all requests related
to the TMF will have been completed by the time the TMF itself completes,
the request queue's callback might not have run yet.  This causes requests
to be completed more than once, and as a result triggers a variety of
BUGs or oopses.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Venkatesh Srinivas <venkateshs@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>