drivers: xen: dom0: added getvcpuinfo domctl call

Domain control call "XEN_DOMCTL_getvcpuinfo" provides information
about domain's virtual CPU(s). It can be used by Domain-0 services
for gathering statistic and vCPUs current status.

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