qmp.py: Couple of pylint/style fixes

No actual code changes, just initializing attributes earlier to avoid
AttributeError on early introspection, a few pylint/style fixes and
docstring clarifications.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170818142613.32394-7-ldoktor@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>