libgeom: Improve type safety of xml2tree code

When resolving references, assert that the type of the object we find
is what we expect. This will help prevent memory corruption if two
objects of different types somehow end up with the same identifier.

MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D54454

(cherry picked from commit 4b0d5d1d6a7c9773e38882feb4747a76b37a645c)