This contains backports for code changes made during in-tree driver
instance snapshot development [1].
- Renames boot_disk_path_for_instance() to get_bootdisk_path() for
clarity
- Makes disk_match_func() private and static
- Renames instance_disk_iter() to _get_bootdisk_iter() and remove
unnecessary parameters.
- Other minor cleanup
[1] https://review.openstack.org/#/c/543023/
Change-Id: I8f2eef1063b7e40c0cd71fc4adb04cbf6d35ece1
e3b112dc93