builder: add a mandatory 'arch' key in index files

Introduce a mandatory arch= key in all the entries of index files, to
identify which architecture is each. Adapt the long and JSON list
outputs to print also this new field.

This introduces an incompatibility with index files created with
virt-builder < 1.26, as they will be rejected until entries will have
the arch= key added (which is ignored by older virt-builder, so adding
it will not create backward-compatibility issues).
builder: split INI C <-> OCaml glue code in own module

Move in an own module the code which calls the C
virt_builder_parse_index and does the array -> list conversion of the
result. This way this code can be easily called also in places different
than Index_parser without the need to copy the types mapping, etc.

Just code motion, no actual behaviour changes.
sysprep: use Mkdtemp to create the temporary directory

Use the safer mkdtemp instead of manually creating a path.