KVM: selftests: Implement ucall() for s390x

On s390x, we can neither exit via PIO nor MMIO, but have to use an
instruction like DIAGNOSE. Now that ucall() is implemented, we can
use it in the sync_reg_test on s390x, too.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20190731151525.17156-3-thuth@redhat.com


Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>