usb-hid: create & use HIDState

First step in separating out the HID emulation code from usb-hid, so it
can be reused without creating a dummy usb device like bluetooth does.

This creates a HIDState struct, moves the non-usbish fields from
USBHIDStruct there.  Renames non-usbish structs, defines and functions
from usb* to hid*.  Adapts the code to that.

Also cleans up a bunch of code style issues along the way.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>