analyze: Do no require a full d-bus bus for the plot command (#8539)

analyze: Do no require a full d-bus bus for the plot command (#8539)

The plot command requires a full d-bus bus to fetch the host
information, which seems rather optional, and having a running dbus
daemon is not always desirable. So instead, we try to acquire a full
bus, and if that fails we acquire the systemd bus, in which case we
omit the host information from the output.

We refactor acquire_bus() into two new functions which in addition
makes the call sites clearer.