Migrating from IDE to VIRTIO disk for existing disk

For migrating existing disk devices from IDE to VIRTIO, after changing xml file from hda to vda as below guest OS (RHEL5) is not booting, getting as "No VolGroups found" and kernel panic

   
   dev='vda' bus='virtio'/>

Virtualization: KVM

Guest OS: RHEL5 disk configured with LVM

Host OS: RHEL 6.3