scsi-bus: prepare scsi_req_new for introduction of parse_cdb

The per-SCSIDevice parse_cdb callback must not be called if the
request will go through special SCSIReqOps, so detect the special
cases early enough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi-bus: introduce parse_cdb in SCSIDeviceClass and SCSIBusInfo

These callbacks will let devices do their own request parsing, or
defer it to the bus.  If the bus does not provide an implementation,
in turn, fall back to the default parsing routine.

Swap the first two arguments to scsi_req_parse, and rename it to
scsi_req_parse_cdb, for consistency.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi-block: extract scsi_block_is_passthrough

This will be used for both scsi_block_new_request and the scsi-block
implementation of parse_cdb.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi-block, scsi-generic: implement parse_cdb

The callback lets the bus provide the direction and transfer count
for passthrough commands, enabling passthrough of vendor-specific
commands.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
virtio-scsi: implement parse_cdb

Enable passthrough of vendor-specific commands.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>