docs: don't ignore virEvent API

docs: don't ignore virEvent API

Commit 6094ad7b (0.9.3 release) promoted several functions from
internal to public, but forgot to fix the documentation generator
to provide details about those functions.

For an example of what this fixes, look at:
file:///path/to/libvirt/docs/html/libvirt-libvirt.html#virEventAddHandle
before and after the patch.

* docs/apibuild.py (ignored_functions): Don't ignore functions
that were turned into official API.
* src/util/virevent.c: Fix comments to pass through parser.