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
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=922475
Signed-off-by: Liron Aravot <laravot@redhat.com>
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>