Missing PCI "programming interface" ID emulation for USB host controllers

Qemu doesn't currently emulate the correct value of the "register level programming interface" field in the PCI config space of USB host controllers. As a result, some guest operating systems (most notably Windows) fail to load e.g. generic xHCI drivers, and instead ask for a vendor-specific driver, which may not be always available.

Example: "-device nec-usb-xhci" emulates a Renesas (formerly NEC) uPD720202 xHCI controller. However, in the PCI config space, it shows us as class 0C, subclass 03, prog-if 00 (UHCI) where as the real uPD720202 (and all real xHCI controllers) have prog-if 30 (xHCI). A Windows guest booted with this option will not recognize devices attached to the XHCI controller unless drivers from Renesas are manually installed first, even though recent Windows versions include a generic xHCI driver that works perfectly well with real uPD720202 hardware.