KVM: s390: generalize kvm_s390_get_tod_clock_ext()

Move the Multiple-epoch facility handling into it and rename it to
kvm_s390_get_tod_clock().

This leaves us with:
- kvm_s390_set_tod_clock()
- kvm_s390_get_tod_clock()
- kvm_s390_get_tod_clock_fast()

So all Multiple-epoch facility is hidden in these functions.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>