restapi: Fix NPE on Jboss startup

Fixed an NPE on Jboss startup caused by trying to get the wrong key from
the stats map, the name was changed along with the StorageDomain entity name
change but was supposed to remain the same since the stat name is hardcoded
in GetSystemStatisticsQuery

Change-Id: I66643eb750dd1f7d7054db387a24a843b211a7c5
Signed-off-by: Tal Nisan <tnisan@redhat.com>