Suspending a domain works with a 3.16 gues kernel but not with a 4.16 one

Suspending a domain with `systemctl suspend` works with a guest 3.16 kernel (jessie), the domain goes into `pmsuspend` in libvirt but doesn't work anymore with a 4.16 one (sid) where:
 - With a QXL card: the spice display just goes black and the domain stays `running` in libvirt but is "dead"
 - With a VGA card: the screen goes black and comes back immediately, the domain stays fine

Qemu: Master, 281bd281222776229d5dbf84d1a5c6d8d9d2a34b