Add DatastoreFlag Stat method
Use DatastoreFlag.Stat method in vmdk.PrepareDestination
Check that DiskFlag.Disk exists

Fixes github issue #90
Don't create VM if vmx already exists

Skip vmx check if -force flag is given
Merge pull request #95 from dougm/datastore-stat

govc: DatastoreFlag.Stat method to check for existing vmdk/vmx files