spapr: fix memory leak in spapr_memory_pre_plug()

The string returned by object_property_get_str() is dynamically allocated.

(Spotted by Coverity, CID 1375942)

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 8a9e0e7b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>