This is the beginnings of support for a `virt-xml --convert-to-vnc`
option. Take an existing VM, strip out most of the previous graphics
config, and add VNC graphics.
We try to convert over some of the shared graphic bits, like listen
and port settings, if they were previously specified.
If spice GL was enabled, we convert to egl-headless config
Signed-off-by: Cole Robinson <crobinso@redhat.com>