monitor: Limit QError use to command handlers

The previous commits narrowed use of QError to handle_qmp_command()
and its helpers monitor_protocol_emitter(), build_qmp_error_dict().
Narrow it further to just the command handler call: instead of
converting Error to QError throughout handle_qmp_command(), convert
the QError gotten from the command handler to Error, and switch the
helpers from QError to Error.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
