virtio_pci: pass vector policy enum to vp_find_vqs_msix()

In preparation for another irq allocation fallback,
introduce vector policy enum and pass the values
to vp_find_vqs_msix() instead of bool arg.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-4-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>