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

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 <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[Split into file and identifier rename, two comments touched up]
Signed-off-by: Markus Armbruster <[email protected]>