Fix: libpe_status: fix precedence of operation meta-attributes

Operations were unpacked such that op_defaults were the base, then any XML
properties of the <op> tag itself took precedence over that, and then any
meta_attributes and instance_attributes beneath the <op> tag had the lowest
precedence.

This changes it so that the <op> tag has the highest precedence,
then <op> <meta_attributes>, then <op_defaults> <meta_attributes>,
then <op> <instance_attributes> (which is deprecated for setting
meta-attributes).