s390/kvm: use union tod_clock

Use union tod_clock and get rid of the kvm specific struct
kvm_s390_tod_clock_ext which apparently was introduced for the same
purpose.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>