qapi: add qapi-commands.py code generator

qapi: add qapi-commands.py code generator

This is the code generator for qapi command marshaling/dispatch.
Currently only generators for synchronous qapi/qmp functions are
supported. This script generates the following files:

  $(prefix)qmp-marshal.c: command marshal/dispatch functions for each
                          QMP command defined in the schema. Functions
                          generated by qapi-visit.py are used to
                          convert qobjects recieved from the wire into
                          function parameters, and uses the same
                          visiter functions to convert native C return
                          values to qobjects from transmission back
                          over the wire.

  $(prefix)qmp-commands.h: Function prototypes for the QMP commands
                           specified in the schema.

$(prefix) is used in the same manner as with qapi-types.py

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@gmail.com>