Attempt to turn on vhost-net mode for devices of type NETWORK, BRIDGE,
and DIRECT (macvtap).

* src/qemu/qemu_conf.h: add vhostfd to qemuBuildHostNetStr prototype
  add qemudOpenVhostNet prototype new flag to set when :,vhost=" found in
  qemu help
* src/qemu/qemu_conf.c: * set QEMUD_CMD_FLAG_VNET_HOST is ",vhost=" found
  in qemu help
   - qemudOpenVhostNet - opens /dev/vhost-net to pass to qemu if everything
     is in place to use it.
   - qemuBuildHostNetStr - add vhostfd to commandline if it's not empty
     (higher levels decide whether or not to fill it in)
   - qemudBuildCommandLine - if /dev/vhost-net is successfully opened, add
     its fd to tapfds array so it isn't closed on qemu exec, and populate
     vhostfd_name to be passed in to commandline builder.
* src/qemu/qemu_driver.c: add filler 0 for new arg to qemuBuildHostNetStr,
  along with a note that this must be implemented in order for hot-plug of
  vhost-net virtio devices to work properly (once qemu "netdev_add" monitor
  command is implemented).