s390: Fix ram_size updating in machine init

s390: Fix ram_size updating in machine init

The global variable 'ram_size' is hidden by the local variable
declaration in s390_init(). Since we want to update the global
ram size in certain cases we must not use a local ram_size
variable.
- This fixes booting with unusual ram sizes like -m 67001
- This changes behaviour back to the situation before commit
  5f072e1f3075bd869e0ace9f2545a85992ac0084
  (create struct for machine initialization arguments)

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>