scsi: Ensure command and transfer lengths are set for all SCSI devices

scsi-generic relies on those values to be correct, so it is important that
those values are initialized properly for all device types.

Reported-by: Christian Hoff <christian.hoff@de.ibm.com>
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
