qapi: rename QmpInputVisitor to QObjectInputVisitor

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

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

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1475246744-29302-5-git-send-email-berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Straightforwardly rebased, split into file and identifier rename]
Signed-off-by: Markus Armbruster <armbru@redhat.com>