ide: add IDEBus struct, cleanups

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.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>