s390/protvirt: Add sysfs firmware interface for Ultravisor information

That information, e.g. the maximum number of guests or installed
Ultravisor facilities, is interesting for QEMU, Libvirt and
administrators.

Let's provide an easily parsable API to get that information.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>