core: fix AddVmParameters for null diskInfoList

In change I06f2de3db1b0559af4df04d0d158b10b8460c684:

 b7804ad core: cleanup AddVmParameters constructors

the logic in charge to ensure that diskInfoList is never null was
removed.

When a new VM is added from rest-api addVmFromScratch(...) sets
diskInfoList to a null value:

  AddVmParameters params = new AddVmParameters(staticVm);
  params.setDiskInfoList(mapDisks(vm.getDisks()));

resulting in a NPE:

 ERROR [AddVmFromScratchCommand] Error during CanDoActionFailure:
         java.lang.NullPointerException
   at checkImagesConfiguration(ImagesHandler.java:455) [bll.jar:]
   at canDoAction(AddVmFromScratchCommand.java:150) [bll.jar:]
   at internalCanDoAction(CommandBase.java:746) [bll.jar:]

Change-Id: I979bd6373e4a1440f88d27e416bd2e116b010891
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>