KVM: s390: get rid of register asm usage

Using register asm statements has been proven to be very error prone,
especially when using code instrumentation where gcc may add function
calls, which clobbers register contents in an unexpected way.

Therefore get rid of register asm statements in kvm code, even though
there is currently nothing wrong with them. This way we know for sure
that this bug class won't be introduced here.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20210621140356.1210771-1-hca@linux.ibm.com
[borntraeger@de.ibm.com: checkpatch strict fix]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>