KVM: s390: Fix epsw instruction decoding

The handle_epsw() function calculated the first register in the wrong way,
so that it always used r0 by mistake. Now the code uses the common helper
function for decoding the registers of rre functions instead to avoid such
mistakes.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>