qapi: Minor introspect.py cleanups

qapi: Minor introspect.py cleanups

Commit 7d0f982b changed generated introspection output to no longer
produce long lines in the generated .c file, but failed to adjust
comments to match.  Add some clarity that the shorter length that
matters most is the overall QMP response on the wire.

Commit 25b1ef31 triggers a pep8 formatting nit.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180827213943.33524-2-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>