Fix: libcrmcommon: properly handle XML comments when comparing v2 patchset diffs

This bug existed since v2 patchsets were created, but did not result in any
known problematic behavior until 1073786e exercised the code differently,
at which point the crmd could get into an infinite election loop when there is
more than one comment at the same level within a CIB XML element, the partition
has only one node, and the partition does not yet have a DC.

It is also possible that ACLs could be bypassed in some cases when changing
only comments.