ui/vnc: fix skipping SASL SSF on UNIX sockets

ui/vnc: fix skipping SASL SSF on UNIX sockets

The 'is_unix' flag is set on the VNC server during startup, however,
a regression in:

  commit 8bd22f477f68bbd7a9c88e926e7a58bf65605e39
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Fri Feb 3 12:06:46 2017 +0000

    ui: extract code to connect/listen from vnc_display_open

meant we stopped setting the 'is_unix' flag when QEMU listens for
VNC sockets, only setting when QEMU does a reverse VNC connection.

Rather than fixing setting of the 'is_unix' flag, remove it, and
directly check the live client socket address. This is more robust
to a possible situation where the VNC server was listening on a
mixture of INET and UNIX sockets.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>