virtio_pci: Fix misleading comment for queue vector

This patch fixes misleading comments in both legacy and modern
virtio-pci device implementations. The comments previously referred to
the "config vector" for parameters and return values of the
`vp_legacy_queue_vector()` and `vp_modern_queue_vector()` functions,
which is incorrect.

Signed-off-by: Liming Wu <liming.wu@jaguarmicro.com>
Message-Id: <20250731092757.1000-1-liming.wu@jaguarmicro.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>