Prepare documentation for existing Xen Domain-0 APIs

[firscity]

Currently Zephyr has a lot of functions in Xen Domain-0 APIs that are barely documented (domctl, sysctl etc.). A lot of them are just wrappers for Xen hypercalls, that are documented in Xen public headers. Howewer, since we are stabilizing zephyr-xenlib (main user of these APIs), we need to provide Doxygen documentation for them right in APIs headers.