s390x/kvm: cleanup partial register handling

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

It made the code more complicated though. During development for life
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>
Acked-by: Alexander Graf <agraf@suse.de>