Add BERI-specific virtio mmio "platform" device.
This device provides a connector to Altera PIO allowing us
to interrupt software-implemented virtio mmio backend.

Sponsored by:	DARPA, AFRL
In vnode_pager_generic_getpages() vp->v_mount is dereferenced in the
beginning, thus can't be NULL.

Sponsored by:	Nginx, Inc.