ide: Register vm change state handler once only

ide: Register vm change state handler once only

We register the vm change state handler in a PCI BAR map() function.
This function can be called multiple times throughout the lifetime of a
PCI IDE device.  This results in duplicate vm change state handlers
being register, none of which are ever unregistered.

Instead, register the vm change state handler in the device's init
function once and for all.

piix tested, cmd646 and via not tested.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>