QEMU supports serving VNC over a unix domain socket rather than traditional
TCP host/port. This is specified with:

<graphics type='vnc' socket='/foo/bar/baz'/>

This provides better security access control than VNC listening on
127.0.0.1, but will cause issues with tools that rely on the lax security
(virt-manager in fedora runs as regular user by default, and wouldn't be
able to access a socket owned by 'qemu' or 'root').

Also not currently supported by any clients, though I have patches for
virt-manager, and virt-viewer should be simple to update.

v2:
    schema: Make listen vs. socket a <choice>