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These settings are necessary for Xen and enabling them here, using the
same mechanism that is used by k8s in this layer, simplifies building
images for Xen.

- Ensure that Xen mode is enabled in QEMU.
- On ARM platforms: enable Flattened Device Tree (FDT) support.
Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>