Refactoring work for the copy-offload/volume populator implementation.

Fixes:

• Fix the pure rest-client - tiny change in GET query param

Changes included:

• Reduce the selector to a factory function - use a function instead of a struct with fields to initialize and return a populator by disk type
• Remove unused function
• Encapsulate populator fields - don't export fields when not needed
• Remove vmware.Client duplicates
• Inline usage of settings for populator selection

Branch: volume-to-volume
Commits: 2de719e4d..92667171e