- fix some log messages

- fix some log messages
- get usage count for USB (currently, just the usage count for
  usb_storage); if in use, don't try to unload the HCD drivers. This is
  an ugly workaround; the real solution is to fix all the HCD drivers to
  shutdown properly in the kexec path, and not unload them manually
  ever. A node will have problems if it both boots from USB, and its HCD
  shares an IRQ with another device. So far, it's been either one or the
  other.