block: Add node name to BLOCK_IO_ERROR event

The event currently only contains the BlockBackend name. However, with
anonymous BlockBackends, this is always the empty string. Add the node
name so that the user can still see which block device caused the event.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>