One API adds an exact device based on bus+dev, the other adds
any device matching vendor+product

* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Add new
  qemuMonitorAddUSBDeviceExact() and qemuMonitorAddUSBDeviceMatch()
  commands.
* src/qemu/qemu_driver.c: Switch over to using the new
    qemuMonitorAddUSBDeviceExact() and qemuMonitorAddUSBDeviceMatch()