core: possible NPE in UpdateVmDisk

During UpdateVmDisk command, if the disk doesn't exist anymore in the
db the private commands lists won't be initialized causing to
NPE on getSharedLocks() method.

Change-Id: I787b4a9e723e3096b5d22a5a925cd976940497ff
Signed-off-by: Liron Aravot <laravot@redhat.com>