Extend parser and add support for USB devices in QEmu/KVM

Extend parser and add support for USB devices in QEmu/KVM
* src/domain_conf.c src/domain_conf.h src/qemu_conf.c
  src/qemu_driver.c: Patch from Guido Günther allowing to pass
  usb devices to qemu/kvm
* docs/libvirt.rng: add the new functionality to the grammar
* tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-address.args
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-address.xml
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-product.args
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-product.xml
  tests/qemuxml2argvtest.c tests/qemuxml2xmltest.c: adding examples
  to the regression tests
* libvirt.spec.in: fix the licence tag
Daniel