xen/intr: correct misuses of Xen handle pointer type

Fix a few spots where handle pointers were incorrectly used. Luckily
these appear rarely triggered given how long they've been lurking.

Fixes: 76acc41fb7c7 ("Implement vector callback for PVHVM and unify event channel implementations")
Fixes: 9f40021f288c ("Introduce a new, HVM compatible, paravirtualized timer driver for Xen.")
MFC after: 2 weeks
Reviewed by: royger