[qtdbus] Don't leak pending call objects when peer disconnects

Unlike in regular connection to DBus server, we don't get pending call
notifies when a peer drops the connection in peer-to-peer mode.

Thus, we need to keep track of pending calls in such cases and get rid of
them in ~QDBusConnectionPrivate().

Signed-off-by: Kalle Vahlman <kalle.vahlman@movial.com>