Radhakrishna,
You must have a bootable image for the install procedure steps to work. That means you will need either physical media -or- a UCOSInstall_<version>.iso. The ISO images on CCO are not bootable. Cisco cannot provide bootbale images to customer for download because of licensing reasons. Once your CUCM is installed in VM, the you can use any CCO CUCM ISO to upgrade (that is fine).
To speed up the process, sometimes customers will create a bootable ISO image from their physical media to make the mapping of the virtual CD easier.
Thanks,
Dan Keller
Technical Marketing Engineer