Several fixes, allowing stress tests to pass:
- fix broken deviceWasAllocated logic in _AddNewDevice(), fixing the
  crash observed before.
- start only once our UsbRoster at first context init, and stop it at last exit.
- add trace to track UsbRoster's DeviceAdded/Removed() : seems that
BUSBRoster:DeviceRemoved() is only called on hub nodes!?
Fixed hotplug issue.
BUSBRoster will only notify device removal for those where DeviceAdded()
returned B_OK.
This make libusbx hotplug works under Haiku too.