RFC: Kernel virtiofs driver

Hi everyone,

    I recently finished the virtiofs driver and it is now ready for review.
The device allows for sharing directories between a FreeBSD guest and a
host.

The driver really is two components:

1) The virtio device that sends FUSE tickets to and from the host: D46295
<https://reviews.freebsd.org/D46295>
2) The file system that gets mounted in the guest: D46296
<https://reviews.freebsd.org/D46296>.

To test it you need a couple additional fixes/workarounds for FUSE-related
issues. You can grab a working tree here
<https://github.com/etsal/freebsd-src/tree/virtiofs> or apply diffs D55047
<https://reviews.freebsd.org/D55047> and D55046
<https://reviews.freebsd.org/D55046>. D55046 is a workaround, but still
prevents an assertion failure related to FUSE caching until the underlying
issue is properly fixed on HEAD.

To use it, make sure you are creating virtiofs device on the host then from
the FreeBSD guest run

mount -t virtiofs <tag> <mountpoint>

where <tag> is the name tag you gave to the virtiofs device in the host VMM.

Reviews and testing welcome!

Thanks,
Emil