core, engine: Fix VM CD change in JSON RPC

This change fixes a NPE while changing the CD of a running VM.

Change-Id: I976354fca483d1cdb45ba7abc006924eb7041af7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1145728
Signed-off-by: Vitor de Lima <vdelima@redhat.com>