qapi: add qapi-visit.py code generator

qapi: add qapi-visit.py code generator

This is the code generator for qapi visiter functions used to
marshal/unmarshal/dealloc qapi types. It generates the following 2
files:

  $(prefix)qapi-visit.c: visiter function for a particular c type, used
                         to automagically convert qobjects into the
                         corresponding C type and vice-versa, and well
                         as for deallocation memory for an existing C
                         type

  $(prefix)qapi-visit.h: declarations for previously mentioned visiter
                         functions

$(prefix) is used as decribed for qapi-types.py

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