fix bug when defining descriptor types

[river-li]

When we tried to add backward compatibility on bndb files, we added a function get_type, but the result of

bv.get_code_refs_for_type("EFI_PEI_NOTIFY_DESCRIPTOR")

and

bv.get_code_refs_for_type(bv.types("EFI_PEI_NOTIFY_DESCRIPTOR")

is different.

The latter is empty and thus fails to define those descriptors.

Fixed this bug by not supporting defining descriptor types on existing bndb anymore. (Consider supporting this in the native rewriting version)