builder: Update TODO file with some more ideas.
builder: Allow checksum= as an alias for checksum[sha512]=.

This is for backwards compatibility with virt-builder 1.24.0 which
could not parse a key that contains square brackets and numbers.

This updates commit 4b73e0be29302da0ae8c5c79a51e770cbe6ed7cf.
builder: Only warn about unknown fields in the index if -v option is used.

Allows us to add new fields in future without having pointless
warnings from old versions of virt-builder.