qapi: return a 'missing parameter' error

qapi: return a 'missing parameter' error

The 'old' dispatch code returned a QERR_MISSING_PARAMETER for missing
parameters, but the qapi qmp_dispatch() code uses
QERR_INVALID_PARAMETER_TYPE.

Improve qapi code to return QERR_MISSING_PARAMETER where
appropriate.

Fix expected error message in iotests.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20160930095948.3154-4-marcandre.lureau@redhat.com>
[Drop incorrect error_setg() from qmp_input_type_any() and
qmp_input_type_null()]
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>