build-sys: move qmp-introspect per target

The following patches are going to introduce per-target #ifdef in the
schemas.

The introspection data is statically generated once, and must thus be
built per-target to reflect target-specific configuration.

Drop "do_test_visitor_in_qmp_introspect(&qmp_schema_qlit)" since the
schema is no longer in a common object. It is covered by the per-target
query-qmp-schema test instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190214152251.2073-7-armbru@redhat.com>