virtio_blk: allow re-reading config space at runtime

Wire up the virtio_driver config_changed method to get notified about
config changes raised by the host.  For now we just re-read the device
size to support online resizing of devices, but once we add more
attributes that might be changeable they could be added as well.

Note that the config_changed method is called from irq context, so
we'll have to use the workqueue infrastructure to provide us a proper
user context for our changes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>