Finalize xHCI and USB modules

Scratchpad buffers allocation added to xHCI. Missing initialization steps
for keyboards - set configuration and set idle - added to USB. (Even
though they are specific to HID protocol, they go through USB command
endpoint, and with current module contents it's cleaner to have them
in USB module.)

src/kernel/hid-keyboard.c

@@ -170,6 +170,7 @@ static uint64_t stream_read(__attribute__((unused)) stream_t *self, uint64_t max
   }
   buffer_length -= size;
   buffer_offset = (buffer_offset + size) % BUFFER_SIZE;
+
   return size;
 }