* src/qemu/qemu_conf.c: Remove separator from qemuBuildNicStr()
  args, and remove hardcoded 'nic' prefix. Leave it upto callers
  instead
* src/qemu/qemu_driver.c: Switch over to using the new
  qemuMonitorAddPCINetwork() method for NIC hotplug
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Add new
  qemuMonitorAddPCINetwork API for PCI network device hotplug