snippets: add virtual Xen dom0 snippet support

Zephyr snippets allow to apply pre-defined build configuration and
extentions to your build0. It is made by applying generic snippet
conf/device-tree changes and may be extended with platform-specific
files.

It looks like great approach for convertion Zephyr OS build to Xen
control domain on different boards, as it requires Kconfig changes
and board device tree changes (configuration for memory node, hypervisor
console, grant table region etc).

Please note, that Xen hypervisor passes all selected parameters via
domain device-tree, but Zephyr does not use it in runtime. So, user
should keep values in board overlays in sync with real Xen values.

Add example and docs for converting qemu_cortex_a53 board to Xen
Domain-0 guest. This approach might be used for other boards, where
Zephyr Dom0 is needed.

Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>
Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>