monitor: Clean up handle_hmp_command() a bit

Leave checking qobject_type(req) to qmp_check_input_obj().  Rework
handling of json_parser_parse_err() failing without setting an error.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-14-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
