CLOUDSTACK-2379: Fix for issue destroy vm causing NPE. Made the following changes.
1. Made a fix to make sure a null object is added to the exception.
2. Also fixed the marvin test cases for the feature. Account cleanup will remove the vms deployed for the account. There is no need to explicitly delete the vms for the account.
3. Fixed the assertion checks for the vm created for an account. If there are multiple vms for an account, the test script needs to compare the ids with the correct instance.