s390x/kvm: rework KVM synchronize to tracing for some ONEREGS

Some ONE_REGS on s390 are not protected by a capability. Older kernels
might not provide those and return an error. Fortunately these registers
are only critical for the migration path. There is no need to error out
on reset and normal runtime. Furthermore, these kernels don't provide
a proper dirty bitmap anyway, so let's use tracing for those errors.

Also provide generic one reg helper to simplify the code.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
