Use kwargs in all the scsi.<command> functions

Standardize of passing kwargs through the scsi.<function> wrappers
and update the param/return comments to describe all arguments
mandatory as well as optional.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
make decode_bits support multibyte fields

Move decode_bits to the generic ceonverter/helper function.
Enhance it to allow handling multi-byte fields by use of specifying a
mask >0xff. Thus specifying [0xffff, 4] means 'the two bytes starting at
offset 4.

Now that decode_bits can handle multibyte fields we can remove a lot
of manual decodes using scsi_ba_to_int and self.add_result and instead
just let decode_bits handle them.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Merge pull request #32 from sahlberg/misc-updates

Misc updates