libusbhost: It's no longer assumed that "bus/usb" exists once "bus" was created

On some devices there is a slight delay between the creation of "/dev/bus"
and "/dev/bus/usb". Previously, the code assumed that both are created in the
same time which caused "watch_existing_subdirs" to fail and libusbhost to stop
working until the device is rebooted. The fix will setup an inotify event on the
creation of the "bus/usb" so it will not be missed once it's created.

Change-Id: I17f06dd167e61573307425e48898e12ebc954093