man: improve VtableExample

man: improve VtableExample

The methods published by the example have a reply in the signature, but
the code was not sending any, so the client gets stuck waiting for a
response that doesn't arrive. Echo back the input string.

Update the object path to follow what would be the canonical format.

Request a service name on the bus, so that the code can be dropped in a
service and it can be dbus-activatable. It also makes it easier to see
on busctl list.