Enhance convert-constants list generation

While looking at the hypervisor types in Constants.hs, I saw that the
'hyperTypes' list is using strings instead of names. This is due to
the fact that we require the entire elements in the list to be
identified (homogeneous lists), but the string "xen-pvm" is declared
by both 'defaultEnabledHypervisor' and 'htXenPvm'.

We can improve convert-constants by adding a list of names that we
know (statically) don't declare, but instead reuse values. Unless we
have an ADT parser, this is the best we can do, I think.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>