kvm: fix types for cross-debugging

As with other libkvm interfaces use maximum-sized types to support
cross-debugging (e.g. a 64-bit vmcore on a 32-bit host). See
https://lists.freebsd.org/pipermail/svn-src-all/2019-February/176051.html
for further discussion.

This is an API-breaking change, but there are few consumers of this
interface today.

Reviewed by: will
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21945