virtio: fix comments, readability

Fix a couple of comments to match reality.
Initialize config_driver_disabled to be consistent with
other fields (note: the structure is already zero initialized,
so this is not a bugfix as such).

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <7b74a55a5f3dc066d954472f5b68c29022f11b43.1752094439.git.mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>