I have less problems on my slower machines. When I ran my VM's on an X5670, I had zero crashes ever. When I moved them to a 6140, sometimes it would crash 4-5 times a day. Currently I have the VM sitting on an e5-2690v2 and it's been happy for a week now. Previously it did crash once each on an e5-2670v0 and an e5-2670v2.
Heavy graphics seems to be what triggers it. I often trigger it in one of three ways:
1) I preview mkv files in thunar. On the 6140 I would usually crash it in about an hour of fiddling like this.
2) Firefox. Switching tabs, scrolling on a web page, loading new web page. The web page doesn't need to be graphically intense. I've crashed it while scrolling down in netbox just filling out a form.
3) pulling up a VNC from another box and fiddling around in there.
I've never had the vm crash overnight, or when idle, even with firefox running. I've never triggered it by just moving windows around or fiddling about in emacs. It's always heavy graphics use. I haven't tried something like running a demo overnight on it or something. I have 4 nodes of 6140's, it crashes the same on all 4 nodes. No other non-spice VM's crash, on any node, regardless of workload.
The VM that crashes does NOT use virgl. I have a few VM's with virgl, but they are very lightly used, so I cannot say one way or another if they are crash prone.
Worth noting, I believe one of the other replies to this post was experiencing this on a windows VM, whereas mine is debian, so it looks like OS is not a factor.