[RFC] remove vhost-cuse

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

It's something echoed around in my mind for a long while, and here I'm
gonna make it public: a proposal to remove vhost-cuse.

Vhost-cuse was invented before vhost-user exist. The both are actually
doing the same thing: a vhost-net implementation in user space. But they
are not exactly the same thing.

Firstly, vhost-cuse is harder for use; no one seems to care it, either.
Furthermore, since v2.1, a large majority of development effort has gone
to vhost-user. For example, we extended the vhost-user spec to add the
multiple queue support. We also added the vhost-user live migration at
v16.04 and the latest one, vhost-user reconnect that allows vhost app
restart without restarting the guest. Both of them are very important
features for product usage and none of them works for vhost-cuse.

You now see that the difference between vhost-user and vhost-cuse is
big (and will be bigger and bigger as time moves forward), that you
should never use vhost-cuse, that we should drop it completely.

The remove would also result to a much cleaner code base, allowing us
to do all kinds of extending easier.

A talk with Huawei offline showed that he backs this proposal. I was
also told by Ciara that she actually had the same idea: she has already
cooked a patch to remove vhost-cuse support from OVS:

    http://openvswitch.org/pipermail/dev/2016-July/074696.html

So I'm proposing to mark vhost-cuse as deprecated in this release and
remove it completely at the next release (v16.11).

Comments/thoughts, or objections?

	--yliu