s390/os_info: Store virtual memory layout

This is a preparatory rework to allow uncoupling virtual
and physical addresses spaces.

The virtual memory layout will be read out by makedumpfile,
crash and other user tools for virtual address translation.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>