qmp.py: Avoid "has_key" usage

The "has_key" is deprecated in favor of "__in__" operator.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170818142613.32394-9-ldoktor@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>