* src/qemu/qemu_conf.h, src/qemu/qemu_conf.c: Remove prefix arg
  from qemuBuildHostNetStr which is no longer required
* src/qemu/qemu_driver.c: Refactor to use qemuMonitorAddHostNetwork()
  API for adding host network
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Add new
  qemuMonitorAddHostNetwork() method for adding host networks