Support VirtIO network interface

[ytakano]

Support VirtIO network interface.

• specification: https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html
• PCIe's matching function (vendor ID): https://github.com/openbsd/src/blob/48010215788d25a5802613d7ba22d657ef6b7049/sys/dev/pci/virtio_pci.c#L356
• attach: https://github.com/openbsd/src/blob/48010215788d25a5802613d7ba22d657ef6b7049/sys/dev/pv/if_vio.c#L622
• osdev
  • PCI: https://wiki.osdev.org/PCI
  • PCIe: https://wiki.osdev.org/PCI_Express
  • VirtIO: https://wiki.osdev.org/Virtio