bus: s/sd_message_handler_t/sd_bus_message_handler_t/g
bus: rename sd_bus_get_peer() to sd_bus_get_server_id()

This function always returns the server side ID. The name suggested it
was actually always the peer's ID, but that's not correct if the call is
called on a server bus context. Hence, let's correct the name a bit.
journal-gatewayd: query PID 1 for virtalization

Since journal-gatewayd is now running unprivileged, and detecting
virtalization requires privileges, query PID1 via D-Bus for the used
virtualization.

This is also the first time we use libsystemd-bus for more than just
testing.

https://bugs.freedesktop.org/show_bug.cgi?id=62173