Move libvirt NFS volume driver impl into volume.py

For inexplicable reasons the NFS volume driver was put into a
different file than all the other libvirt volume driver impls.
The NFS code isn't so large & complex that it needs to be in
a different place than everything else. Move the code into
the main volume.py file.

To avoid breaking existing deployments, the original class
has to be a trivial subclass of the new module. Any use of it
will trigger a deprecation warning. This allows time for users
to update their 'libvirt_volume_drivers' config (in the unlikely
event they have altered from the default setting).

Change-Id: I26d957e4ee33938b45dc1cad0fe32a2968520a4a
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Move libvirt volume driver tests to separate test case

The libvirt volume code is in a separate module from the
main libvirt driver code, so it makes sense for the test
suite to be separated too, to make it easy to quickly
run just the volume tests. This has the added advantage
of reducing the size of the enourmous test_libvirt.py
file which is becoming unwieldly.

Change-Id: I7c6134992055694798c4ec3217483bc3a41d89eb
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Move libvirt NFS volume driver impl into volume.py"
Merge "Move libvirt volume driver tests to separate test case"