XML: upgrade-2.10.xsl: operation's "requires" [2/2]: fix plain conflicts

Here, "plain conflicts" means:

• overriding of existing (primitive|template)/meta_attributes[ not(rule) ]/nvpair[@name='requires'] from either (primitive|template)/operations/op/@requires or (primitive|template)/operations/op/meta_attributes[ not(rule) ]/nvpair[ @name='requires' ] [see test-2/023-rsc-requires-no-override]

• mutual clash between formerly stated sources, following the rules:
  • op/@X takes precedence over op/meta_attributes/nvpair[name='X']
  • last occurrence within either class takes precedence over other members of that class [see test-2/024-rsc-requires-no-selfclash]

Note that presence of "rule" inside meta_attributes shall always be
propagated as is, without evaluation of the conflicts.