qapi: New classes QAPIGenC, QAPIGenH, QAPIGenDoc

qapi: New classes QAPIGenC, QAPIGenH, QAPIGenDoc

These classes encapsulate accumulating and writing output.

Convert C code generation to QAPIGenC and QAPIGenH.  The conversion is
rather shallow: most of the output accumulation is not converted.
Left for later.

The indentation machinery uses a single global variable indent_level,
even though we generally interleave creation of a .c and its .h.  It
should become instance variable of QAPIGenC.  Also left for later.

Documentation generation isn't converted, and QAPIGenDoc isn't used.
This will change shortly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180211093607.27351-6-armbru@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[eblake: fix nits spotted by Michael]
Signed-off-by: Eric Blake <eblake@redhat.com>