vringfd()

The immediate use-case is to allow a high performance virtual network device backend to be
implemented in userspace for KVM.

In general, it's just a standardized ring queue between kernel and userspace.  Ring queues are
lock-less and efficient when shared between two CPUs.  They are good at batching and
implementing zero-copy IO.

vringfd() will be most immediately useful for tun/tap users.  Of course, it's easy to envision
a vringfd() interface for block IO.