virt-v2v: No root device found in this operating system image

when i import vm.img file file in RHEV by virt-v2v utility, i am gettng error "No root device found in this operating system image".

virt-v2v -i libvirtxml -o rhev -osd 172.20.10.80:/data/ovirt --network rhevm VM_libvirt.xml

vm.img: 100% [=======================================================]D 0h00m35s
virt-v2v: No root device found in this operating system image.

i have already install libguestfs-winsupport and virtio-win package even though i am getting this error..