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-before-err.json b/tests/qapi-schema/include-before-err.json
new file mode 100644
index 0000000..afb6cb6
--- /dev/null
+++ b/tests/qapi-schema/include-before-err.json
@@ -0,0 +1,2 @@
+{ 'include': 'include-simple-sub.json' }
+{ 'command' 'missing-colon' }