Description of problem:
guest VM qemu processes will occasionally become defunct (requiring host restart) due to the enabling of experimental_zcopytx in the vhost_net module when using macvtap

Version-Release number of selected component (if applicable):
kernel-3.8.5-201.fc18.x86_64

How reproducible:
Use a KVM Windows 7 VM with macvtap & bridging (vhost_net driver) + netkvm, run network traffic and/or attempt to restart the guest

Actual results:
guest transfers network data / restarts without kernel warning and defunctness

Expected results:
kernel warning, defunct process
(abrt reports: https://retrace.fedoraproject.org/faf/reports/69052/)
(see the abrt BZ ticket: https://bugzilla.redhat.com/show_bug.cgi?id=918015)

Additional info:
The problem appears to be solved for me by adding

/etc/modprobe.d/vhost_net.conf:
options vhost_net experimental_zcopytx=0

...maybe the option should be distribution-disabled?