Support Virtiofs (New filesystem type based on FUSE)

For heavy containers running full linux (like cloud-hypervisor) there's an option to provide filesystem using virtiofs protocol (which is FUSE where the server is running on the host).

Would be helpful if gVisor also supported that.

Applications are obvious: a container may want to access large filesystems without downloading it on the host (as both root and non-root filesystems).

(We actually only need readonly access with overlay, which is probably much easier to make work, but I guess others may want full rw access.)

Currently fuse can be mounted on the host and exported to gVisor as regular host filesystem, but it has drawbacks:
- larger overhead
- less secure
- easier to overwhelm host kernel
- bugs depend host kernel version

There's document describing gVisor FUSE work in progress ([fuse.md](https://github.com/google/gvisor/blob/master/pkg/sentry/vfs/g3doc/fuse.md)), but it only mentions fuse server running in the sandbox, but not on the host.