bhyve: Add virtio modern support handling

[khng]

Summary

This adds virtio modern support handling to bhyve.
pci_emul_add_capability is exposed to support PCIY_VENDOR-type
virtio modern capabilities.

Current virtio devices are slightly changed to adopt
vi_pci_cfgread/vi_pci_cfg_write.

Virtio modern is not exposed to device emulation code currently.

Sponsored by: The FreeBSD Foundation

[khng]

Rebased after virtio-input support was added

[khng]

VS_LOCK/VS_UNLOCK in vi_interrupt causes deadlock in case MSI-X is disabled.

[corvink]

What's the state of this patch? Does it still works?