We currently start the bus manager only after the creation of a
NMManager because the NMManager is needed to handle set-property bus
calls. However, objects created by NMManager
(e.g. NMDnsSystemdResolved) need a bus connection and so their
initialization currently fail.

To fix this, split nm_dbus_manager_start() in two parts: first only
create the connection and acquire the bus.  After this step the
NMManager can be set up. In the second step, set NMManager as the
set-property handler and start exporting objects on the bus.

Fixes: 297d4985