Investigate use of os.copy_file_range for NFS

[Eric Harney]

The NFS driver currently calls shutil.copyfile() in _copy_volume_image().

Python 3.8 now supplies an os.copy_file_range() method:
https://docs.python.org/3.8/library/os.html#os.copy_file_range

This should allow offloading this copy operation to the NFS server when using a new enough environment.

shutil may do this itself eventually, but it doesn't yet:
https://bugs.python.org/issue37159