Various tidying changes accumulated during that effort. They're all
functionally independent afaict, but there may be contextual
interactions between some of them.

v2 addresses review comments, which however goes as far as introducing
a new patch (2).

1: paging: fold most HAP and shadow final teardown
2: paging: drop set-allocation from final-teardown
3: paging: move update_paging_modes() hook
4: paging: move and conditionalize flush_tlb() hook
5: shadow: drop zero initialization from shadow_domain_init()

Jan