Add exception handling for copy_volume_to_image()

The method require_driver_initialized occurs exception so that the
method volume_get will be not executed, and the variable 'volume'
will be not defined. In the "finally" code segment, the
volume['instance_uuid'] will get the UnboundLocalError, this patch
gets volume before require_driver_initialized to avoid the above
problem.

Change-Id: Ic8167383eb67c5016c9853da274455e0f202dc4d
Closes-Bug: #1322958
Merge "Add exception handling for copy_volume_to_image()"