scsi: Rename scsi_*_length() to scsi_*_xfer(), add scsi_cdb_length()

scsi_cdb_length() does not return the length of the cdb, but
the transfersize encoded in the cdb. So rename it to scsi_cdb_xfer()
and also rename all other related functions to end with _xfer.

We can then add a new scsi_cdb_length() which actually does return the
length of the cdb.  With that DEBUG_SCSI can now display the correct
CDB buffer.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>