PPC: strengthen PReP-picking algorithm (v3)

[Will Woods]

This version of the patch fixes a potential misfire if bootDevice() is called
before partitioning is committed - we could have a "create" action with a
later "destroy" action, which would render the earlier "create" moot.

The differences from the previous patch are:

-            if a.format.type == "prepboot":
+            if a.format.type == "prepboot" and a.device in
self.storage.devices:

...which works because (if I understand dlehman correctly) devices that are
*actually* going to be created by the requested actions will be in the
self.storage.devices list.. and I removed a stray footnote from the commit
message.

-w