KVM: s390: add kvm reference to struct sie_page2

Adding the kvm reference to struct sie_page2 will allow to
determine the kvm a given gisa belongs to:

  container_of(gisa, struct sie_page2, gisa)->kvm

This functionality will be required to process a gisa in
gib alert interruption context.

Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Message-Id: <20190131085247.13826-11-mimu@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>