qemu.py: Pylint/style fixes

No actual code changes, just several pylint/style fixes and docstring
clarifications.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20170818142613.32394-2-ldoktor@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>