xen: refactor Xen hypervisor Kconfig options

Xen-related Kconfig options were highly dependand on BOARD/SOC xenvm.
It is not correct because Xen support may be used on any board and SoC.

So, Kconfig structure was refactored, now CONFIG_XEN is located in
arch/ directory (same as in Linux kernel) and can be selected for
any Cortex-A arm64 setup (no other platforms are currently supported).

Also remove confusion in Domain 0 naming: Domain-0, initial domain,
Dom0, privileged domain etc. Now all options related to Xen Domain 0
will be controlled by CONFIG_XEN_DOM0.

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