qapi: rename *qmp-*-visitor* to *qobject-*-visitor*

The QMP visitors have no direct dependency on QMP. It is
valid to use them anywhere that one has a QObject. Rename them
to better reflect their functionality as a generic QObject
to QAPI converter.

This is the first of three parts: rename the files.  The next two
parts will rename C identifiers.  The split is necessary to make git
rename detection work.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Split into file and identifier rename, two comments touched up]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/scripts/qapi-event.py b/scripts/qapi-event.py
index 38d8211..6d9a36d 100644
--- a/scripts/qapi-event.py
+++ b/scripts/qapi-event.py
@@ -209,7 +209,7 @@
 #include "qemu-common.h"
 #include "%(prefix)sqapi-event.h"
 #include "%(prefix)sqapi-visit.h"
-#include "qapi/qmp-output-visitor.h"
+#include "qapi/qobject-output-visitor.h"
 #include "qapi/qmp-event.h"
 
 ''',