Fix startup with VNC password expiry on old QEMU

Fix startup with VNC password expiry on old QEMU

The code which set VNC passwords correctly had fallback for
the set_password command, but was lacking it for the
expire_password command. This made it impossible to start
a guest. It also failed to check whether QEMU was still
running after the initial 'set_password' command completed

* src/qemu/qemu_hotplug.c: Fix error handling when
  password expiry fails
* src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_text.c: Fix
  return code for missing expire_password command