s390: get rid of odd global real_memory_size

The variable real_memory_size has odd semantics and has been used in
a broken way by e.g. the old kvm code.
Therefore get rid of it before anybody else makes use of it.

Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>