qemu.py: Simplify QMP key-conversion

The QMP key conversion consist of '_'s to be replaced with '-'s, which
can easily be done by a single `str.replace` method which is faster and
does not require `string` module import.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170818142613.32394-5-ldoktor@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>