[efi] Process veto objects in reverse order of enumeration

[efi] Process veto objects in reverse order of enumeration

While not guaranteed by the UEFI specification, the enumeration of
handles, protocols, and openers will generally return results in order
of creation.  Processing these objects in reverse order (as is already
done when calling DisconnectController() on the list of all handles)
will generally therefore perform the forcible uninstallation
operations in reverse order of object creation, which minimises the
number of implicit operations performed (e.g. when disconnecting a
controller that itself still has existent child controllers).

Signed-off-by: Michael Brown <mcb30@ipxe.org>