oem_ibm: Fix for dump offload issues

This commit is to fix the dump offload issue by using
GetManagedObjects instead of GetSubTreePaths to search for the
dump entry match.

Tested by triggering the system dump and resource dump offloads.

Signed-off-by: Jayashankar Padath <jayashankar.padath@in.ibm.com>
Change-Id: Ic85980b6fdc4030ace48d03d7cef335eae69d34f