xen: add Kconfig option for Zephyr on Dom0less setups

Dom0less is Xen mode without privileged domain. All guests are created
according to hypervisor device tree configuration on boot. Thus, there
is no Dom0 with console daemon, that usually manages console output
from domains.

Zephyr OS contains 2 serial drivers related to Xen hypervisor: regular
with console shared page and consoleio-based. The first one is for
setups with console daemon and usually was used for Zephyr DomU guests.
The second one previously was used only for Zephyr Dom0 and had
corresponding Kconfig options. But consoleio is also used as interface
for DomU output on Dom0less setups and should be configurable without
XEN_DOM0 option.

Add corresponding XEN_DOM0LESS config to Xen Kconfig files and proper
dependencies in serial drivers.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>
Co-authored-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>