Fix node volumes list for stripped volumes

Currently backend.NodeVolumes() drops everything except the first PV,
thus we get a truncated result. The patch is not the nicest, as Python
doesn't have a simple `concat' function, so I had to change the list
comprehension to an explicit loop.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>