manager: create a virtual device only if the connection can autoconnect
The autoconnection for virtual devices currently works in two phases. First we detect that there is suitable profile that can autoconnect and we realize the device. Then, when the device becomes 'disconnected', autoconnect kicks in and starts the activation.
However, if autoconnect is blocked for a device, currently we do step 1 without step 2, leaving a stale interface around. Fix this by also checking that autoconnect is not blocked during step 1.