QMP: Introduce qmp_check_input_obj()

This is similar to qmp_check_client_args(), but it checks if
the input object follows the specification (QMP/qmp-spec.txt
section 2.3).

As we're limited to three keys, the work here is quite simple:
we iterate over the input object, checking each time if the
current argument complies to the specification.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
