The device_add command was added in JSON mode in a way I didn't
expect. Instead of passing the normal device string to the JSON
command:

    { "execute": "device_add", "arguments": { "device": "ne2k_pci,id=nic.1,netdev=net.1" } }

We need to split up the device string into a full JSON object

    { "execute": "device_add", "arguments": { "driver": "ne2k_pci", "id": "nic.1", "netdev": "net.1" } }

* src/qemu/qemu_conf.h, src/qemu/qemu_conf.c: Rename the
  qemuCommandLineParseKeywords method to qemuParseKeywords
  and export it to monitor
* src/qemu/qemu_monitor_json.c: Split up device string into
  a JSON object for device_add command