Description of problem:
could not set mtu > 1500 when using virtio-net on SLES 10 SP4 guest

Version-Release number of selected component (if applicable):
Host:
qemu-img-rhev-0.12.1.2-2.415.el6.x86_64
kernel-2.6.32-428.el6.x86_64
Guest:
# uname -r
2.6.16.60-0.85.1-default

How reproducible:
100%

Steps to Reproduce:
1. boot SLES 10 SP4 guest with virtio-net nic
/root/staf-kvm-devel/autotest-devel/client/tests/virt/qemu/qemu \
    -S \
    -name 'virt-tests-vm1' \
    -nodefaults \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/tmp/monitor-qmpmonitor1-20131105-232306-xg7IopJV,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control \
    -chardev socket,id=serial_id_serial1,path=/tmp/serial-serial1-20131105-232306-xg7IopJV,server,nowait \
    -device isa-serial,chardev=serial_id_serial1 \
    -chardev socket,id=seabioslog_id_20131105-232306-xg7IopJV,path=/tmp/seabios-20131105-232306-xg7IopJV,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20131105-232306-xg7IopJV,iobase=0x402 \
    -device ich9-usb-uhci1,id=usb1,bus=pci.0,addr=0x4 \
    -drive file='/root/staf-kvm-devel/autotest-devel/client/tests/virt/shared/data/images/sles10-64-virtio.qcow2',index=0,if=none,id=drive-virtio-disk1,media=disk,cache=none,snapshot=off,format=qcow2,aio=native \
    -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk1,bootindex=0 \
    -device virtio-net-pci,netdev=idmO1hp9,mac='9a:f4:f5:f6:f7:f8',bus=pci.0,addr=0x6,id='idtrVx6E' \
    -netdev tap,id=idmO1hp9,vhost=on,vhostfd=25,fd=24 \
    -m 2048 \
    -smp 2,maxcpus=2,cores=1,threads=1,sockets=2 \
    -cpu 'Penryn' \
    -M rhel6.5.0 \
    -device AC97,addr=0x7 \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1 \
    -vnc :0 \
    -vga cirrus \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off   \
    -no-kvm-pit-reinjection \
    -enable-kvm
2. change Ethernet mtu > 1500 in guest
ifconfig eth6 mtu 65520
3.

Actual results:
set failed, error info:
"SIOCSIFMTU: Invalid argument"

Expected results:
set mtu=65520 ok

Additional info:
1. rtl8139, e1000 can set successfully
rtl8139: max mtu=1500
e1000: max mtu=16110