Fix XML serializer for QoS Specs extension

This patch fixes XML serializer for QoS Specs exension calls like
list(), show(), associations().  Note that using XML as input for
create(), update() doesn't support yet.

Closes-Bug: # 1229619

Change-Id: I2eab9d69e05bb03269b51731458994d6268ac852
Merge "Fix XML serializer for QoS Specs extension"