Print idx <-> uuid mappings in machine readable mode

As per Iustin Pop's suggestion, we are printing group mappings
from group idx (known only to htools) to group uuid, which identifies
the group uniquely. This is required because group uuid and name
may contain characters such as '-' which are not valid parts of
bash variable name.

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>