This internal method isn't used by any code other than tests.

Signed-off-by: Joshua Lock <jlock@vmware.com>

The call stack and code for download_target() is more complex than
required:
* download_target() : builds target destination filepath, gets length
  and hashes
* _get_target_file() : fixes filenames if consistent snapshots enabled,
  defines verification callback
* _get_file() : iterates mirrors, tries to download files, verifies them

Remove the verification callback and collapse the call stack by a single
level to make the code easier to follow.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Rather than read to the end of the file in order to determin its size, use
the whence value of seek() to move the file object's position to the end
of the file, then the tell() method of the file object to read the current
position in bytes.

Co-authored-by: Jussi Kukkonen <jkukkonen@vmware.com>
Signed-off-by: Joshua Lock <jlock@vmware.com>

Simplify the loop exit logic in _get_target_file() to simply return a
verified file_object, once we have it, rather than breaking from the loop
and then returning the file_object.

This converts a use of a try/except/else to a try/except and is a little
easier to read.

Signed-off-by: Joshua Lock <jlock@vmware.com>