vhost: add note about sockets in server mode

vhost: add note about sockets in server mode

From time to time, someone sends patches about unlinking existing
sockets when registering a vhost user in server mode.

A recent example:
	http://dpdk.org/ml/archives/dev/2018-February/090025.html

This problem has been discussed many times, and it was made clear that
the library should not unlink files given by the application in order
to avoid possible security problems, such as removing random files
used by other programs.

One of the first discussions:
	http://dpdk.org/ml/archives/dev/2015-December/030326.html

To avoid such patches in the future, it was decided to add a comment
that explains what is happening and tries to describe the reasoning.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>