mgr/vol: don't define progress bar ID repeatedly

mgr/vol: don't define progress bar ID repeatedly

Orignally, when the feature was in development, IDs for clone progress
bars were set to randomly generated UUID strings. But, eventually, it
was decided to assign fixed strings to them. Unlike UUIDs, these strings
stay the same even when progress bars are destroyed and re-created.
Therefore, instead of re-assigning the same strings every time
initiate_reporting() is called, move them to __init__() so that both the
IDs are defined only once.

Signed-off-by: Rishabh Dave <ridave@redhat.com>