modemmanager: do not cache virtual device events

modemmanager: do not cache virtual device events

On small systems with many virtual devices, the modem manager sometimes
could not start because it took too long until all devices for the modem
were recognised. This is because all system events that are stored in
the file events.cache have to be processed. To speed up the processing,
all devices under /sys/devices/virtual are now filtered out so that they
do not have to be processed.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>