With the next patch, a device may hold SCSIRequest for an indefinite
time. Split a rather big patch, and protect against access errors,
by reference counting them.
There is some ugliness in scsi_send_command implementation due to
the need to unref the request when it fails. This will go away
with the next patches, which move the unref'ing to the devices.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Cc: Christoph Hellwig <hch@lst.de>