core: fix the merging of identical objects in presence of Misc objects

If object A contains B + I/O as children, we can "ignore" I/Os and still
try to merge A and B. We now do the same for Misc objects without cpusets
instead of I/Os.

This fixes a corner case when export/reimport to XML creates a slightly
different topology (making hwloc_insert_misc fail inside a Linux cgroup).

Thanks to Dave Love for reporting the problem.

Fixes #118

(cherry picked from commit 650371e1159fb3826afae8842545c4d16bdad1c8)