add virtio file system device support

[emil_etsalapatis.com]

Summary

Supporting the virtiofs requires a device driver for the virtio fs device. Add the virtio file system driver before introducing the file system itself.

[lwhsu]

sys/dev/virtio/fs/virtio_fs.h

5

We need a copyright holder here, and may drop "all rights reserved."

[asomers]

sys/dev/virtio/fs/virtio_fs.c

59

This line is redundant with the one in virtio_fs.h

297

551–552

sys/dev/virtio/fs/virtio_fs_internal.h

11

If these messages are just for debugging purposes, shouldn't there be a way to turn them off?

[emil_etsalapatis.com]

address reviewer feedback

[emil_etsalapatis.com]

• sys/dev/virtio/fs: drain virtqueues one at a time

[lwhsu]

sys/dev/virtio/fs/virtio_fs.h

3

BSD-2-Clause is fine, no need -FreeBSD suffix

[emil_etsalapatis.com]

• sys/dev/virtio/fs: use regular BSD-2-Clause license