KVM: s390: Support Configuration z/Architecture Mode

KVM: s390: Support Configuration z/Architecture Mode

BugLink: http://bugs.launchpad.net/bugs/1719297
kvm has always supported the concept of starting in z/Arch mode so let's
reflect the feature bit to the guest.

Also, we change sigp set architecture to reject any request to change
architecture modes.

Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
(cherry picked from commit b697e435aeee99b7f5b2d8f8dbb51f791be99b16)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>