We chain up the Generic plugin created MMBroadbandModem objects within the
GDBusObjectManagerServer in MMManager, so that they get properly exported in
DBus.
The new mm_plugin_manager_is_finding_port_support() allows to check whether the
Plugin Manager is looking for support in a specific port.
The new mm_plugin_manager_is_checking_device_support() allows to check whether
the Plugin Manager is looking for support in any port of a given device.
The new `mm_plugin_manager_find_port_support()' method requests the Plugin
Manager to iterate over the list of plugins internally handled, launching
supports task for the given port in each of them.
The method is fully asynchronous, and the result can be retrieved with
`mm_plugin_manager_find_port_support_finish()' once the operation is ready.