Remove datastore test files

Any govc-test- files or directories that are not taken care of by
vm.destroy
Output disk file backing in device.info
Add vm disk related bats tests
FindByBackingInfo -> SelectByBackingInfo

Add support for file based backing.
Add VirtualDeviceList.FindSCSIController method
Add FindDiskController helper
Add helpers for creating disks
Add VirtualDisk support to device helpers

{Add,Edit,Remove}Device methods need a FileOperation
Change disk related commands to use new helpers
Merge pull request #158 from dougm/device-helpers

Additional disk related device helpers