Get virtio notify offset multiply from PCI configuration

Created by: tjhu

We are hard coding it to 4 because this https://github.com/qemu/qemu/blob/d55f518248f263bb8d0852f98e47102ea09d4f89/hw/virtio/virtio-pci.c#L226

It's either 4 or 4k.

We should read it from the PCI configuration instead https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-1150004