Refactor: libcrmcommon: move validation to apply_upgrade()

Previously, apply_upgrade() applied the XSL transforms but did not verify the
result validated. update_validation() called apply_upgrade() then verified the
result.

It makes more sense to do the validation in apply_upgrade() since the upgrade
did not succeed if the result does not validate. This changes the return value
from pcmk_err_schema_validation to pcmk_err_transform_failed, but that actually
makes more sense.