KVM: s390: Handle hosts not supporting s390-virtio.

KVM: s390: Handle hosts not supporting s390-virtio.

commit 55c171a6d90dc0574021f9c836127cfd1a7d2e30 upstream.

Running under a kvm host does not necessarily imply the presence of
a page mapped above the main memory with the virtio information;
however, the code includes a hard coded access to that page.

Instead, check for the presence of the page and exit gracefully
before we hit an addressing exception if it does not exist.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>