Reorganize usb source files.  Create a new hw/usb/ directory and move
all usb source code to that place.  Also make filenames a bit more
descriptive.  Host adapters are prefixed with "hch-" now, usb device
emulations are prefixed with "dev-".  Fixup paths Makefile and include
paths to make it compile.  No code changes.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>