Hi
let me give a little background first. mkinitrd script may call lvm. Sometimes this call fails when the lvm version in the distro converted is different from the one in helper VM. To work around this issue we patch the initrd image by injecting the lvm (and its libraries) from the helper, run mkinitrd, and then restore the original binaries in the image. All this requires additional disk space and time unfortunately.
This stuff happens in the /boot directory which is being created in a separate partition at the destination (even if it is not at the source). If you increase the boot partition size before starting the conversion, this should work around the issue. As an approximation, I suppose having it 3 times the size of the initrd image should suffice. In your case you have 32% usage, which is in the expected size. I think it hasn't been enough by just a small amount. But if you want to be on the safe side, double the partition size.
HTH
Plamen