Description of problem:
Run "ethtool -K eth0 tso on" failed when boot a guest using "-net tap,vlan=0,fd=21"

# ethtool -K eth0 tso on
Cannot set device tcp segmentation offload settings: Invalid argument
# echo $?
88

Version-Release number of selected component (if applicable):
2.6.18-304.el5

How reproducible:
100%

Steps to Reproduce:
1. boot a guest with "-net tap,vlan=0,fd=21"

2. turn on tso with command:
ethtool -K eth0 tso on

Actual results:
the command got an error:
Cannot set device tcp segmentation offload settings: Invalid argument

Expected results:
turn on tso

Additional info:
1. execute "echo $?" after "ethtool -K eth0 tso on", I can get the value 88

2. boot a guest with script rather than fd, everything is ok.
-net tap,vlan=0,script=/etc/qemu-ifup-switch