Let's validate the given address and report a proper error in case it's
not. All call paths now properly check the validity of the given GFN.
Remove the TODO.

The errors inside the getter and setter should only trigger if something
really goes wrong now, for example, with a broken migration stream. Or
when we forget to update the storage key allocation with memory hotplug.

Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210903155514.44772-12-david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>