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/qapi/qapi-clone-visitor.c b/qapi/qapi-clone-visitor.c
index 0bb8216..34086cb 100644
--- a/qapi/qapi-clone-visitor.c
+++ b/qapi/qapi-clone-visitor.c
@@ -110,7 +110,7 @@
     assert(qcv->depth);
     /*
      * Pointer was already cloned by g_memdup; create fresh copy.
-     * Note that as long as qmp-output-visitor accepts NULL instead of
+     * Note that as long as qobject-output-visitor accepts NULL instead of
      * "", then we must do likewise. However, we want to obey the
      * input visitor semantics of never producing NULL when the empty
      * string is intended.