qxl: dont update invalid area

This patch fixes the following error:

$ ~/usr/bin/qemu-system-x86_64 -enable-kvm -m 1024 -spice port=5900,disable-ticketing -vga qxl -cdrom ~/Images/linuxmint-13-mate-dvd-32bit.iso
(/home/mathslinux/usr/bin/qemu-system-x86_64:10068): SpiceWorker-CRITICAL **: red_worker.c:4599:red_update_area: condition `area->left >= 0 && area->top >= 0 && area->left < area->right && area->top < area->bottom' failed
Aborted

spice server terminates QEMU process if we pass invalid area to it,
so dont update those invalid areas.

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
