s390x: Enable new s390-storage-keys device

s390x: Enable new s390-storage-keys device

s390 guest initialization is modified to make use of new s390-storage-keys
device. Old code that globally allocated storage key array is removed.
The new device enables storage key access for kvm guests.

Cache storage key QOM objects in frequently used helper functions to avoid a
performance hit every time we use one of these functions.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>