s390x/pci: enable uid-checking

The uid-checking facility guarantees uniqueness of the uid within the
vm and exposes the real uid to the guest when listing pci devices.
Let's always enable it and present it to the guest in the response to
the list pci clp command.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
