The current QEMU disk media change does not support setting the
disk format. The new JSON monitor will support this, so add an
extra parameter to pass this info in

* src/qemu/qemu_driver.c: Pass in disk format when changing media
* src/qemu/qemu_monitor.h, src/qemu/qemu_monitor.c,
  src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h:
  Add a 'format' arg to qemuMonitorChangeMedia()