datatypes: Fix comments

datatypes: Fix comments

- As of commit 2ff4c137, all virGet*() functions in datatypes.c always
  return pointers to new objects. Objects are not cached in a
  per-connection hashtable.

- Fix variable names in comments for all vir*Dispose() functions in
  datatypes.c.

- Add comments for virGetStream(), virStreamDispose(),
  virGetDomainSnapshot(), virDomainSnapshotDispose().

Signed-off-by: Michael Chapman <mike@very.puzzling.org>