Refactor: fencer: Match primitive XPath only with ID

An ID is required when schema validation is enabled anyway, and this
allows us to be sure rsc_id will be non-NULL within the block. It also
allows us to shorten some strstr() searches by starting later in the
string, not that this will matter in practice.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>