Each QEMUWin32AIOState event notifier is associated with an AioContext.
Since BlockDriverState instances can use different AioContexts we cannot
continue to use a global QEMUWin32AIOState.

Let each BDRVRawState have its own QEMUWin32AIOState and free it when
BDRVRawState is closed.
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>