The idea is to have a static table that defines the dbus API. The vtable is
defined right next to the interface name and path because they are logically
connected.
Let's define a new, generic bus interface that any daemon can implement
for querying/setting the log level.
We can turn this into something more powerful later on, but for now,
only expose three properties: the log level, log target and the syslog
identifier (with the former two being writable).
This is supposed to be generic, so that it can be implemented by 3rd
party daemons too, eventually.
Yu Watanabe