Looks like RHEL5 viostor driver will always fail while trying to start on RHEL6.
One possible workaround would be:
- start VM on rhel6 with "cache=writethrough" option,
- update viostor driver with the latest one from http://download.lab.bos.redhat.com/devel/RHEV/virtio-win/1.1.8-0/,
- shutdown VM,
- turn cache back to "off",
- start VM.

Verify win7 guest with virtio-win-1.1.8-0, this bug does not exist.
I verified this bug as Jes and Vadim steps:
1)install win7 guest on RHEL5.5 host with virtio-win-1.1.8-0
1.run win7 guest above on RHEL6.0 with cli:
/usr/libexec/qemu-kvm -m 2G -smp 2 -cpu qemu64,+x2apic  -usbdevice tablet -drive file=/root/win7-32.qcow2.el5,if=none,id=drive-virtio0,boot=on,werror=stop,rerror=stop,cache=none -device virtio-blk-pci,drive=drive-virtio0,id=virtio-blk-pci0  -netdev tap,id=hostnet0,vhost=on,script=/etc/qemu-ifup -device virtio-net-pci,netdev=hostnet0,mac=00:00:14:3F:20:1f,bus=pci.0 -uuid a4f4127c-e6e3-4e67-95da-8dd0a8891cc4  -name win7-32  -qmp tcp:0:4445,server,nowait   -monitor stdio -vnc :1
2.do restart and shutdown, then start this guest again, this bug does not exist,win7 guest can be run smoothly.

2)using win7 guest installed on RHEL5.5 host with RHEL5 viostor driver.
1.Start win7 guest above RHEL6.0 with cli:
/usr/libexec/qemu-kvm   -m 2G -smp 2 -cpu qemu64,+x2apic  -usbdevice tablet -drive file=/mnt/win7-32-virtio.qcow2,if=none,id=drive-virtio0,boot=on,werror=stop,rerror=stop,cache=writethrough -device virtio-blk-pci,drive=drive-virtio0,id=virtio-blk-pci0 -netdev tap,id=hostnet0,script=/etc/qemu-ifup -device virtio-net-pci,netdev=hostnet0,mac=02:00:16:3F:20:1f,bus=pci.0 -uuid 1234127c-1233-4e67-95da-8dd0a8891cc4  -name win7x64  -qmp tcp:0:4444,server,nowait  -boot c -monitor stdio -boot c -vnc :1
2.update virtio blk driver to virtio-win-1.1.8-0
3.quit this guest
4.start this guest on RHEL6.0 again with cli:
/usr/libexec/qemu-kvm   -m 2G -smp 2 -cpu qemu64,+x2apic  -usbdevice tablet -drive file=/mnt/win7-32-virtio.qcow2,if=none,id=drive-virtio0,boot=on,werror=stop,rerror=stop,cache=none -device virtio-blk-pci,drive=drive-virtio0,id=virtio-blk-pci0 -netdev tap,id=hostnet0,script=/etc/qemu-ifup -device virtio-net-pci,netdev=hostnet0,mac=02:00:16:3F:20:1f,bus=pci.0 -uuid 1234127c-1233-4e67-95da-8dd0a8891cc4  -name win7x64  -qmp tcp:0:4444,server,nowait  -boot c -monitor stdio -boot c -vnc :1
This guest can be start successfully.

So this bug has been resolved.

Event posted on 08-20-2010 09:31am EDT by jkachuck

------- Comment From anantyog.com 2010-08-20 03:49 EDT-------
This issue still occurs with virtio drivers in Snap10 supplement CD
Info about the RPM installed:
rpm -qi virtio-win-1.0.0-8.2.41879.el6.noarch
Name        : virtio-win                   Relocations: (not relocatable)
Version     : 1.0.0                             Vendor: Red Hat, Inc.
Release     : 8.2.41879.el6                 Build Date: Tuesday 06 July
2010 04:29:36 PM EDT
Install Date: Friday 20 August 2010 02:33:08 AM EDT      Build Host:
x86-003.build.bos.redhat.com
Group       : Applications/System           Source RPM:
virtio-win-1.0.0-8.2.41879.el6.src.rpm
Size        : 17125341                         License: Red Hat
Proprietary
Signature   : RSA/8, Thursday 08 July 2010 02:10:16 PM EDT, Key ID
938a80caf21541eb
Packager    : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
URL         : http://www.redhat.com/
Summary     : VirtIO para-virtualized drivers for Windows(R)
Description :
VirtIO para-virtualized Windows(R) drivers for 32-bit and 64-bit
Windows(R) guests.



This event sent from IssueTracker by jkachuck 
 issue 1207333

Same as in BZ#609838, virtio-win-1.0.0 is known to be bad, they need
1.1.8 or later.

Jes

Red Hat Enterprise Linux 6.0 is now available and should resolve
the problem described in this bug report. This report is therefore being closed
with a resolution of CURRENTRELEASE. You may reopen this bug report if the
solution does not work for you.

*** Bug 754921 has been marked as a duplicate of this bug. ***

Created attachment 570916 [details]
BSOD viostor.sys

---Problem Description---
We are trying to migrate an old Windowsw 2008 R2 image with old virtio drivers
from RHEL5.6 to RHEL6.1. The old drivers are not compatible with RHEL6.1


Contact Information = Red Hat

Reverse Mirror of 754921
Hi Mihai,

Could you please upload memory dump file together with VM command line?

Best regards,
Vadim.
(In reply to comment #3)
> ---Problem Description---
> We are trying to migrate an old Windowsw 2008 R2 image with old virtio drivers
> from RHEL5.6 to RHEL6.1. The old drivers are not compatible with RHEL6.1
>
>
> Contact Information = Red Hat
>
> Reverse Mirror of 754921

Is this Bug related to sysprep ? I tried following steps ,can not reproduce

1.install windows 2008R2 host with virtio-win-1.0.3-1.52454.el5.noarch.rpm
CLI#/usr/libexec/qemu-kvm -no-hpet -no-kvm-pit-reinjection -usbdevice trtc-td-hack -startdate now -name win2k8R2 -smp 2 -k en-us -m 2048 -boot dc -net nic,vlan=0,macaddr=00:11:4b:40:19:01,model=virtio -net tap,vlan=0,ifname=virtio_tap0,script=/etc/qemu-ifup,downscript=no -drive file=/home/win08R2.qcow2,media=disk,if=virtio,cache=none,boot=on,format=qcow2,werror=stop -monitor stdio -spice host=0,ic=on,port=5930,disable-ticketing -qxl 1
2.install virtio-netkvm and virtio-blk driver during guest installation
3.copy the image to RHEL6u2 host
4.start the image on RHEL6 host
5.make a copy-on-write image based on the image in step4
#qemu-img create -f qcow2 win08R2.sp1 -b win08R2.qcow2
6.start guest on RHEL6 host with the image created in step 5

Actual Results???
after step 4 ,guest could start successfully ,no BOSD happened
after step 6 ,guest could start successfully ,no BSOD happened .

Hello ,IBM
Could you give more details how I can reproduce this bug ?

Thanks,
Mike
Sounds exactly like the following one:
https://bugzilla.redhat.com/show_bug.cgi?id=609831

Let's close it.
*** This bug has been marked as a duplicate of bug 609831 ***