cmdlib: Add utility for instance data import/export

Interpreting the backend's import/export daemon status is a bit tricky.
This utility code keeps track of multiple transfers at the same time.
Users can supply callback functions to react to events.

Timeouts are currently hardcoded. Intra-cluster instance moves will likely
require other timeouts.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>