We have no known user of real-space designation and only support it to
be architecture compliant.

Gmap shadows with real-space designation are never unshadowed
automatically, as there is nothing to protect for the top level table.

So let's simply limit the number of such shadows to one by removing
existing ones on creation of another one.
Acked-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>