We will keep a reference to each of the support checks currently in progress,
grouped by physical device path. The stored SupportsInfo structs as well as
the support check operations, are guaranteed to be kept valid as long as the
Plugin Manager exists. Or in other words, the Plugin Manager cannot be disposed
if there is still an ongoing supports check operation.
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.
Aleksander Morgado