commit 628eb9b8
    KVM: s390: streamline memslot handling

introduced kvm_s390_vcpu_get_memsize. This broke guests >=4G, since this
function returned an int.

This patch changes the return value to a long.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>