qemu vnc auth 3/4: Add VNC auth support from upstream QEMU

qemu vnc auth 3/4: Add VNC auth support from upstream QEMU

This patch adds in the upstream QEMU VNC authentication code. This
spports the previous VNC password auth scheme, as well as the VeNCrypt
protocol extension. The latter allows for performing a TLS handshake,
and client verification of the server identify using x509
certificates. It is also possible for the server to request a client
certificate and validate that as a simple auth scheme. The code
depends on GNU TLS for SSL APIs, and the configure script will
auto-detect this.

The image.py code is changed to  deal with the new syntax for the -vnc
option.  In particular password authentication is now enabled by
giving the 'password' flag,

eg -vnc 0.0.0:1,password

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>