qapi/commands.py: Don't re-bind to variable of different type

Mypy isn't a fan of rebinding a variable with a new data type.
It's easy enough to avoid.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201009161558.107041-22-jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>