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/tests/test-string-output-visitor.c b/tests/test-string-output-visitor.c
index 444844a..e736db3 100644
--- a/tests/test-string-output-visitor.c
+++ b/tests/test-string-output-visitor.c
@@ -4,7 +4,7 @@
  * Copyright (C) 2012 Red Hat Inc.
  *
  * Authors:
- *  Paolo Bonzini <pbonzini@redhat.com> (based on test-qmp-output-visitor)
+ *  Paolo Bonzini <pbonzini@redhat.com> (based on test-qobject-output-visitor)
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.