virtio_pci: simplify vp_request_msix_vectors() call a bit

Pass desc arg as it is always to vp_request_msix_vectors(). There rely
on per_vq_vectors arg and null desc in case it is false.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-3-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>