kvm tools: Stop guest if SDL window is closed

kvm tools: Stop guest if SDL window is closed

Send SIGKVMSTOP signal to the process if the SDL window is closed. This fixes
an annoying problem where closing the window makes the guest seem like it froze
although it's really running in the background but the UI is unusable.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>