s390x/cpumodel: we are always in zarchitecture mode

s390x/cpumodel: we are always in zarchitecture mode

In QEMU, a guest VCPU always started in and never was able to leave
z/Architecture mode. Now we have an architected way of showing this
condition.

The SIGP SET ARCHITECTURE instruction is simply rejected. Linux as guest
seems to not care about the return value, which is a good thing
The new handling is just like already being in z/Architecture mode.

We'll not try to fake absence of this facility, but still not indicate
the facility in case some strange CPU model turned z/Architecture off
completely (which doesn't work either way but let's us see how a
guest would react on a lack of this facility).

Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>