Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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.

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 after: 1 week