xen: recognize device_model_override

xen: recognize device_model_override

Since Xen 4.2 libxl expects device_model_override="/path" instead of
device_model="/path". Adjust the code to parse this as <emulator>.

While libxl also recognizes device_model_version="", this knob is not
required for libvirt. A runtime detection exists in libvirt to select
either "qemu-xen" or "qemu-xen-traditional".
Since qemu-xen-traditional is marked as supported just for stubdoms
there is no need to handle it.

Test data files with 'device_model' were adjusted to use
'device_model_override' instead.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>