When instantiating an emulated VGA device for an x86 HVM guest qemu
will by default enable a backend to expose that device, either SDL
or VNC depending on the version of qemu and the build time
configuration.
The libxl toolstack library does not explicitly disable these
default backends when they are not enabled, leading to an unexpected
backend running.
If either SDL or VNC is explicitly enabled in the guest
configuration then only the expected backends will be enabled.
This affects qemu-xen and qemu-xen-traditional differently.
If qemu-xen was compiled with SDL support then this would result in
an SDL window being opened if $DISPLAY is valid, or a failure to
start the guest if not.
If qemu-xen was compiled without SDL support then qemu would
instead start a VNC server listening on ::1 (IPv6 localhost) or
127.0.0.1 (IPv4 localhost) with IPv6 preferred if available. A VNC
password will not be configured even if one is present in the guest
configuration.
qemu-xen-traditional will never start a vnc backend unless
explicitly configured. However by default it will start an SDL
backend if it was built with SDL support and $DISPLAY is valid.