s390/dump: fix os_info virtual vs physical address confusion

Due to historical reasons os_info handling functions misuse
the notion of physical vs virtual addresses difference.

Note: this does not fix a bug currently, since virtual
and physical addresses are identical.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>