Host: 
- Debian Linux with kernel 3.11-2-amd64
- QEMU emulator version 1.7.0 (Debian 1.7.0+dfsg-2)
- Spice server 0.12.4
- libvirt Virtual Machine Manager 0.9.5

Guest:
- Ubuntu Saucy with kernel 3.11.0-20-generic

Reproduce:
- Start qemu with option -global qxl-vga.revision=4, which allows drm to load the 
  qxl driver
  Other parameters related to graphics:
  + -device qxl-vga,id=video0,ram_size=67108864,vram_size=67108864,bus=pci.0,addr=0x2
  + -spice port=5900,addr=127.0.0.1,disable-ticketing,seamless-migration=on
  + -chardev spicevmc,id=charchannel0,name=vdagent
  + -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=com.redhat.spice.0
- Check with dmesg, that QXL DRM has been correctly initialised
- Send Ctrl-Alt-Fn (n = 1, 2, ...) to VM guest

Bug:
Virtual machine manager will switch to console but freezes with strange display. Guest cannot be contacted any more. Switching back to vc is not possible. ssh connections will hang. No netconsole output. The guest must be restarted using forced shutdown.

The problem occurs also for qemu 2.0. It does not occur, if only userspace driver is used (omit option -global qxl-vga.revision=4). In this case lspci reports:
00:02.0 VGA compatible controller: Red Hat, Inc. Device 0100 (rev 03)
and dmesg reports:
[drm] Initialized drm 1.1.0 20060810
[drm:qxl_pci_probe] *ERROR* qxl too old, doesn't support client_monitors_config, use xf86-video.

I can provide further information, but need some guidance, what you prefer in order to locate the root cause of the problem.