Low: libcrmcommon: don't validate with first schema for an unknown schema name

Simplify pcmkvalidate_xml() by using pcmkget_schema(). If the schema is
"none", validate_with() will return TRUE, so we don't need to special-case
that.

This approach fixes a bug: previously, for unknown schema names,
get_schema_version() would have returned -1, and g_list_nth_data() would have
returned the head of the list, so we would attempted validation with the first
schema rather than log an error and return FALSE.