screendump with qxl + spice shows stale data

The monitor 'screendump' command returns stale data for VMs using qxl + spice. If you perform multiple screendumps, screendump #N will show roughly the display from the time screendump #N-1 was taken. This affects 'virsh screenshot' and libvirt screenshot APIs by association.

Gerd explains that new monitor commands/infrastructure is likely required to handle this correctly:

https://lists.gnu.org/archive/html/qemu-devel/2014-04/msg03840.html