qmp: Simplify monitor_qmp_respond()

qmp: Simplify monitor_qmp_respond()

monitor_qmp_respond() takes both a response object and an error
object.  If an error object is non-null, the response object must be
null, and the response is built from the error object.

Of the two callers, one always passes a null response object, and one
a null error object.  Move building the response object from the error
object to the latter, and drop the error object parameter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-27-armbru@redhat.com>