Fix explicit vmod dump

[wismill]

xkbcommon already allow pure virtual modifiers (see #450) via explicit mapping (see @mahkoh comment). But it does not roundtrip, because we do not dump the virtual modifier mappings.

This PR enable dumping the explicit mappings. The following issue may be dealt with in a follow-up MR:

The assignment of pure virtual modifiers to bits is implicit. If both parties don't agree on the order in which virtual modifiers are assigned, they cannot communicate. With the mapping explicitly spelled out, it is at least possible for a client to detect the desired bit offset.