bladeRF-cli: Add support for selecting XB-200 filters

This addresses issue #288.
bladeRF-cli: Changed xb argument order and xb command code cleanup

The xb command has been updated such that the arguments are now
in a more intuitive order:

    xb <model_number> <subcommand> [arguments]

Where model_number is "100" or "200" and the subcommand and associated
arguments are dependent upon the XB model.

Fixed detection of invalid xb command arguments and reorganized
xb commands code to improve readability. Further touched up help text.
bladeRF-cli: Added information and touchups to xb command help

The board_model and subcommand items are now annotated with <> to
denote they are mandatory.

Descriptions of each of the filter XB-200 filter options have been
added.
libbladeRF: Added additional information about XB filter & path options
libbladeRF: Touched up logic and debug messages in xb_attach()

After some discussion, disabling or switching XBs is not going
to be supported (currently).  Hotswapping is not recommended, nor
would this be a common use-case.

As such, this is enforced, and the messages have been reduced to
the debug level to generate less noise.