s390x: save/restore cr0 in IRQ handlers

We want to check for certain conditions (e.g. no AFP enabled), but still
have it correctly set up (AFP enabled) in the IRQ handles. So let's
save + restore.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180824115059.1517-4-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>