qa: fix RemoveInstanceOfflineNode for file-based

- The cleanup should only happen for non externally mirrored disk
  templates
- It should behave differently for file and lvm based instances
- It should definitely *not* be explicit in the qa file :/ (but it's
  more important to fix qa, as of now)

Also fix a typo in the same function's docstring.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>