Store VNC auth scheme per-client as well as per-server

Store VNC auth scheme per-client as well as per-server

A future patch will introduce a situation where different
clients may have different authentication schemes set.
When a new client arrives, copy the 'auth' and 'subauth'
fields from VncDisplay into the client's VncState, and
use the latter in all authentication functions.

* ui/vnc.h: Add 'auth' and 'subauth' to VncState
* ui/vnc-auth-sasl.c, ui/vnc-auth-vencrypt.c,
  ui/vnc.c: Make auth functions pull auth scheme
  from VncState instead of VncDisplay

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>