qapi: Split error.json off common.json

In my "build everything" tree, changing a type in qapi/common.json
triggers a recompile of some 3600 out of 6600 objects (not counting
tests and objects that don't depend on qemu/osdep.h).

One common dependency is QapiErrorClass: it's used only in in
qapi/error.h, which uses nothing else, and is widely included.

Move QapiErrorClass from common.json to new error.json.  Touching
common.json now recompiles only some 2900 objects.

Cc: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190812052359.30071-4-armbru@redhat.com>

qapi/error.json

@@ -0,0 +1,29 @@
+# -*- Mode: Python -*-
+
+##
+# = QMP errors
+##
+
+##
+# @QapiErrorClass:
+#
+# QEMU error classes
+#
+# @GenericError: this is used for errors that don't require a specific error
+#                class. This should be the default case for most errors
+#
+# @CommandNotFound: the requested command has not been found
+#
+# @DeviceNotActive: a device has failed to be become active
+#
+# @DeviceNotFound: the requested device has not been found
+#
+# @KVMMissingCap: the requested operation can't be fulfilled because a
+#                 required KVM capability is missing
+#
+# Since: 1.2
+##
+{ 'enum': 'QapiErrorClass',
+  # Keep this in sync with ErrorClass in error.h
+  'data': [ 'GenericError', 'CommandNotFound',
+            'DeviceNotActive', 'DeviceNotFound', 'KVMMissingCap' ] }