qtdbus: Export org.freedesktop.DBus.Peer interface when introspected

qtdbus: Export org.freedesktop.DBus.Peer interface when introspected

The ofD.Peer interface is automatically implemented when an object is
registered on DBus, but the ofD.Introspectable.Introspect method does
not show it.

You can test it by running

$ qdbus --literal <some service> <some object> \
>    org.freedesktop.DBus.Peer.Ping

$ qdbus --literal <some service> <some object> \
>    org.freedesktop.DBus.Peer.GetMachineId

Change-Id: I7062ba0cdae486d443011ee19cf3874aabf90205
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>