monitor: simplify invalid_qmp_mode()

handle_qmp_command() will switch to use qmp_dispatch().  It won't have a
pointer to the marshaller function anymore, but only the name of the
command to invoke. Simplify invalid_qmp_mode() so it can just be called
with the command name.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20160912091913.15831-5-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
