Draft: mtp: use libusb hotplug events to avoid polling new mtp device

The mtp service discoveries is polling every 5 seconds new mtp device. This instead check for new device when a usb device is plugged/unplugged.

This still needs some more testing:

• outside of Linux
• on linux that uses eudev (libusb_open() fail when re-plugging a device on there)
• test the fallback for platforms that doesn't support libusb hotplugging capabilities