ui: convert VNC startup code to use SocketAddress

The VNC code is currently using QemuOpts to configure the
sockets connections / listeners it needs. Convert it to
use SocketAddress to bring it in line with modern QAPI
based code elsewhere in QEMU.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>