Future patches will create devices long before they are backed by
kernel resources, so we need to split NMDevice object creation from
actual setup based on the backing resources.

This patch combines the NMDeviceFactory's new_link() and
create_virtual_device_for_connection() class methods into a single
create_device() method that simply creates an unrealized NMDevice
object; this method is not expected to fail unless the device is
supposed to be ignored.  This also means that the NMDevice
'platform-device' property is removed, because a platform link
object may not be available at NMDevice object creation time.

After the device is created, it is then "realized" at some later
time from a platform link (for existing/hardware devices via the
realize() method) or from an NMConnection (for newly created software
devices via the create_and_realize() NMDeviceClass methods).

https://bugzilla.gnome.org/show_bug.cgi?id=737458