s390x/kvm: cleanup partial register handling

The partial register handling (introduced with commits 420840e58b85f7f4e5493dca3f273566f261090a
3474b679486caa8f6448bae974e131370f360c13 ) aimed to improve intercept handling performance.

It made the code more complicated though. During development for life migration
migration/init/reset etc it turned out that this might cause several hard to debug programming
errors. With the introduction of ioeventfd (and future irqfd patches) the qemu intercept
handlers are no longer hot-path. And therefore the partial register handling can be
removed to simplify the code.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
CC: Jason J. Herne <jjherne@us.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>