build: Deal with all of QAPI's .o in qapi/Makefile.objs

build: Deal with all of QAPI's .o in qapi/Makefile.objs

Adding QAPI's .o to util-obj-y, common-obj-y and obj-y is spread over
three places: Makefile.objs takes care of target-independent generated
code, Makefile.target of target-dependent generated code, and
qapi/Makefile.objs of (target-independent) hand-written code.

Do everything in qapi/Makefile.objs.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190214152251.2073-8-armbru@redhat.com>