There are more operations than a SCSI bus can handle, besides completing
commands.  One example, which this series will introduce, is cleaning up
after a request is cancelled.

More long term, a "SCSI bus" can represent the LUNs attached to a
target; in this case, while all commands will ultimately reach a logical
unit, it is the target who is in charge of answering REPORT LUNs.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>