During guestagent volume operations the calls to unmount check if the
volume is mounted first. The python function os.path.ismount returns
False if the directory if not readable by the current user, breaking
this functionality for some datastores.
The symptom of this failure is that the device ends up mounted twice
during prepare and then fails to unmount fully during resize.
The fix is to create a custom is_mount function that runs as the root user.
Change-Id: I151402717386230371bafcedc170d70b3588e912
Closes-Bug: #1645773