The current IDE code uses an array of two IDEState structs to maintain
the IDE bus. This patch adds a IDEBus to be used instead and does a
bunch of cleanups:
* move ide bus state from IDEState to IDEBus.
* drop a bunch of ugly pointer arithmetics to figure the active
interface, explicitly save the interface number instead.
* add helper functions to save/restore idebus state.
It also fixes a save/restore bug: loadvm allways stores the command in
the master's IDEState, even when it was saved from the slave.