testcases: Exit mainloop instead of calling sys.exit()

Without this, bluetoothd will not terminate properly.
mgmt: Use human readable UUIDs
mgmt: Support for more packets
Initial support for L2CAP sockets
Basic support for L2CAP packets on kernel emulator

With this, the gatt.py testcase will finally be able to run using kernel
emulator.

There are still some fixes to be done on the emulator in order to avoid
several "invalid file descriptor" errors.