Improve meta-virt-xen.inc by moving all the Raspberry Pi 4 config
settings to a new xen-raspberrypi4-64.inc file in the raspberrypi
dynamic layer.

The new .inc file is included only for the raspberrypi4-64 MACHINE,
so the config settings no longer need to be applied using overrides.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>