I started to debug this a little while ago but I never got too far.
However it doesn't seem connected with kvm -- it is something to do
with tun/tap or bridging holding a lock too long I think.
Try running your guest with -no-kvm (and even with the kvm module not
loaded, just to be sure). In my case I still saw the messages.
However, removing the "-net tap" line from my command line did get rid
of the messages.