qemu: Add usb_boot.py test

1) Add a USB device to a QEMU VM
2) Verify that the device shows up on 'info usb' monitor command
3) Verify that the device shows up on 'lsusb -v' shell command
4) Verify that there are no IO Errors showing on dmesg after
   2) and 3) are done.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>