run.py: Do not run Xen guests with user mode networking implicitely

Xen guests do not support user mode networking.
Fall back to qemu when hypervisor detection used and no external
networking configured.

This logic is required for build to support Xen hosts properly.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>