appliance: While building don't just check empty arg, check if it was passed

The fix makes systemvmtemplate as the default veewee vbox appliance if not
other appliance is provided to be built

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>
CLOUDSTACK-1634 VMSnapshot: CreateVMSnapshotCmd fails due to due to java.util.ArrayList cannot be cast to [Lcom.vmware.vim25.ManagedObjectReference