Description of problem:
Could not ping Win7-64 guest successfully after changing e1000 MTU

Version-Release number of selected component (if applicable):
kernel-2.6.32-411.el6.x86_64
qemu-kvm-rhev-tools-0.12.1.2-2.389.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Boot a Win7-64 guest w/ e1000 nic

2. In guest:
   1) wmic nic where macaddress="$guest_macaddress" get netconnectionid  
   2) wmic nic where netconnectionid="Ethernet" get index
   3) reg add HKLM\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}\0001 /v mtu /d 16128 /f 
   4) netsh interface set interface name="Ethernet" admin=DISABLED
   5) netsh interface set interface name="Ethernet" admin=ENABLED 

3. In host:
   1) ifconfig $tap mtu 16128
   2) arp -s $guest_ip $guest_macaddress -i $tap
   3) ping $guest_ip -c 1 -I $tap -s 16100 -M do

Actual results:
Could not ping guest successfully

Expected results:
Ping guest successfully

Additional info:
1. Tested w/ nic rtl8139 & virtio_net successfully.

2. After changing MTU, disable the e1000 driver in guest, then enable it can ping successfully.

3. Qemu CML:
/home/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-qmpmonitor1-20130815-152836-Ih7kA0vX,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control \
    -chardev socket,id=serial_id_serial1,path=/tmp/serial-serial1-20130815-152836-Ih7kA0vX,server,nowait \
    -device isa-serial,chardev=serial_id_serial1 \
    -chardev socket,id=seabioslog_id_20130815-152836-Ih7kA0vX,path=/tmp/seabios-20130815-152836-Ih7kA0vX,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20130815-152836-Ih7kA0vX,iobase=0x402 \
    -device ich9-usb-uhci1,id=usb1,bus=pci.0,addr=0x4 \
    -drive file='/home/staf-kvm-devel/autotest-devel/client/tests/virt/shared/data/images/win7-64.qcow2',index=0,if=none,id=drive-ide0-0-0,media=disk,cache=none,snapshot=off,format=qcow2,aio=native \
    -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0 \
    -device e1000,netdev=idAhvj4X,mac='9a:c0:c1:c2:c3:c4',bus=pci.0,addr=0x3,id='idsiWq2u' \
    -netdev tap,id=idAhvj4X,fd=18 \
    -m 4096 \
    -smp 2,maxcpus=2,cores=1,threads=1,sockets=2 \
    -cpu 'SandyBridge',,+sep \
    -M rhel6.5.0 \
    -drive file='/home/staf-kvm-devel/autotest-devel/client/tests/virt/shared/data/isos/windows/winutils.iso',index=1,if=none,id=drive-ide0-0-1,media=cdrom,format=raw \
    -device ide-drive,bus=ide.0,unit=1,drive=drive-ide0-0-1 \
    -device AC97,addr=0x5 \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1 \
    -vnc :0 \
    -vga std \
    -rtc base=localtime,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off  \
    -enable-kvm

4. cpuinfo:
processor	: 3
vendor_id	: GenuineIntel
cpu family	: 6
model		: 42
model name	: Intel(R) Xeon(R) CPU E31225 @ 3.10GHz
stepping	: 7
cpu MHz		: 1600.000
cache size	: 6144 KB
physical id	: 0
siblings	: 4
core id		: 3
cpu cores	: 4
apicid		: 6
initial apicid	: 6
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
bogomips	: 6184.25
clflush size	: 64
cache_alignment	: 64
address sizes	: 36 bits physical, 48 bits virtual
power management: