qapi: Add a primitive to include other files from a QAPI schema file

The primitive uses JSON syntax, and include paths are relative to the file using the directive:

  { 'include': 'path/to/file.json' }

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/tests/qapi-schema/include-cycle.err b/tests/qapi-schema/include-cycle.err
new file mode 100644
index 0000000..602cf62
--- /dev/null
+++ b/tests/qapi-schema/include-cycle.err
@@ -0,0 +1,3 @@
+In file included from tests/qapi-schema/include-cycle.json:1:
+In file included from include-cycle-b.json:1:
+include-cycle-c.json:1: Inclusion loop for include-cycle.json