KVM: s390: expose gbea register to userspace

For migration/reset we want to expose the guest breaking event
address register to userspace. Lets use ONE_REG for that purpose.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>