MFC r267520, r267521, r267522, r267523, r267524:

- Remove two write-only local variables
- Remove unused element in the vtnet_rxq structure
- Remove kernel specific macro out of the VirtIO PCI header file
- Move the VIRTIO_RING_F_* defines out of virtqueue.h into
virtio_config.h
- Make the feature negotiation code easier to follow
- Force two byte alignment for all control message headers

MFC r266951:

Split the virtio.h header file into multiple files

Reorganize the previous contexts of the file as it is in Linux. The
eventual goal is to install the header files and share them between
the kernel and bhyve.

MFC r267520, r267521, r267522, r267523, r267524:

- Remove two write-only local variables
- Remove unused element in the vtnet_rxq structure
- Remove kernel specific macro out of the VirtIO PCI header file
- Move the VIRTIO_RING_F_* defines out of virtqueue.h into
virtio_config.h
- Make the feature negotiation code easier to follow
- Force two byte alignment for all control message headers

MFC r266951:

Split the virtio.h header file into multiple files

Reorganize the previous contexts of the file as it is in Linux. The
eventual goal is to install the header files and share them between
the kernel and bhyve.