When in JSON mode, QEMU requires that 'qmp_capabilities' is run as
the first command in the monitor. This is a no-op when run in the
text mode monitor

* src/qemu/qemu_driver.c: Run capabilities negotiation when
  connecting to the monitor
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
  src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h: Add
  support for the 'qmp_capabilities' command, no-op in text mode.