display: add -display spice-app launching a Spice client

display: add -display spice-app launching a Spice client

Add a new display backend that will configure Spice to allow a remote
client to control QEMU in a similar fashion as other QEMU display
backend/UI like GTK.

For this to work, it will set up Spice server with a unix socket, and
register a VC chardev that will be exposed as Spice ports. A QMP
monitor is also exposed as a Spice port, this allows the remote client
fuller qemu control and state handling.

- doesn't handle VC set_echo() - this doesn't seem a strong
  requirement, very few front-end use it
- spice options can be tweaked with other -spice arguments
- Windows support shouldn't be hard to do, but will probably use a TCP
  port instead
- we may want to watch the child process to quit automatically if it
  crashed

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Victor Toso <victortoso@redhat.com>
Message-id: 20190221110703.5775-12-marcandre.lureau@redhat.com

[ kraxel: squash incremental fix ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>