libqos: Fixes a small memory leak.

Allow users the chance to clean up the QPCIBusPC structure
by adding a small cleanup routine. Helps clear up small
memory leaks during setup/teardown, to allow for cleaner
debug output messages.

Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
