aaa: Fix ExtMap.get with default value

The implementation before the fix failed to support a case where
the default  value was a subclass of the type associated with the ExtKey

Change-Id: Ib9d04c4650ff0a01866ffd7edf2cce6801d422a1
Topic: AAA
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>