discover: Add uuid and label parameters to persistent device data

discover: Add uuid and label parameters to persistent device data

Rather than depending on the event (which is only available during
inital discovery) for UUID and label parameters, this change adds uuid
and label members to struct discover_device.

This means that the label and UUID are available during the device's
lifetime, not just during initial discovery. We can also just pass the
device to some of the device handling code, rather than the discover
context.

Also, fix an issue where we don't use the raw label/uuid (instead of the
encoded one) in setup_device_links.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>