This patch adds hw/usb-desc.[ch] files.  They carry data structures
for various usb descriptors and helper functions to generate usb
packets from the structures.

The intention is to have a internal representation of the device
desription which is more usable than the current char array blobs,
so we can have common code handle common usb device emulation using
the device description.

The usage of this infrastructure is optional for usb drivers as there
are cases such as pass-through where it probably isn't very useful.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>