qapi: rename QmpOutputVisitor to QObjectOutputVisitor

qapi: rename QmpOutputVisitor to QObjectOutputVisitor

The QmpOutputVisitor has no direct dependency on QMP. It is
valid to use it anywhere that one wants a QObject. Rename it
to better reflect its functionality as a generic QAPI
to QObject converter.

The commit before previous renamed the files, this one renames C
identifiers.

Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>
Message-Id: <1475246744[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[Split into file rename and identifier rename]
Signed-off-by: Markus Armbruster <[email protected]>