The problem is after upgrading your VIOS using the viosupgrade tool, IF your media repository resides on rootvg, which is often the case, it will no longer exist post upgrade. So you can either move it to non-rootvg disks, or save off/copy it’s contents elsewhere and recreate it after the upgrade. I did find additional details on this and it is available at:

https://www.ibm.com/support/pages/viosupgrade-virtual-media-library-lost-after-viosupgrade