s390x/mmu: Use ioctl for reading and writing from/to guest memory

Add code to make use of the new ioctl for reading from / writing to
virtual guest memory. By using the ioctl, the memory accesses are now
protected with the so-called ipte-lock in the kernel.

[CH: moved error message into kvm_s390_mem_op()]
Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
