Description of problem:
When installing OS on guest which have a rtl8139 card via NFS, I found a lot of packets is retransmitted or timeout, and file transmission via NFS is very slow.
Then I changed nic type to e1000, file transfer speed is back to normal.

           public net.                private net with a private bridge.
NFSServer-------------- Host(RHEL5.6)--------------Guest(RHEL5.5)


Version-Release number of selected component:
# rpm -q kernel
kernel-2.6.18-233.el5
# rpm -q kvm
kvm-83-219.el5

How reproducible:
100%

Steps to Reproduce:
1. Start a VM with following command, and start a unattended_install:
qemu -name 'vm1' 
-monitor unix:'/tmp/monitor-humanmonitor1-20101204-045808-YVkj',server,nowait 
-serial unix:'/tmp/serial-20101204-045808-YVkj',server,nowait 
-drive file='RHEL-Server-5.5-64.qcow2',index=0,if=ide,media=disk,cache=none,format=qcow2 
-net nic,vlan=0,model=rtl8139,macaddr='9a:25:00:89:f2:49' 
-net tap,vlan=0,ifname='t0-045808-YVkj',script='qemu-ifup-vbr0',downscript='no' -m 4096 -smp 2,cores=1,threads=1,sockets=2 
-drive file='rhel5-64/ks.iso',media=cdrom,index=1 
-cpu qemu64,+sse2 -soundhw ac97 
-kernel '/usr/local/staf/test/RHEV/kvm-new/autotest/client/tests/kvm/vmlinuz' 
-initrd '/usr/local/staf/test/RHEV/kvm-new/autotest/client/tests/kvm/initrd.img' -vnc :0 -spice port=8000,disable-ticketing -qxl 1 -rtc-td-hack -M rhel5.5.0 
-usbdevice tablet 
-append 'ks=cdrom console=ttyS0,115200 console=tty0' -no-kvm-pit-reinjection

2. tcpdump on the tap device guest created.
3. installation is slow, and many retransmission.
  
Actual results:
file transmission speed is slow, packet retransmission.

Expected results:
nearly no packet retransmission, file transmission speed is normal.

Additional info:
dmesg on guest os is attached.