libxl: store the VDI.attach result in xenstore

We need to know this value so we can predict the xenstore paths
used by the device. We need these paths so we can wait for hotplug.
We need to wait for hotplug because we can't distinguish between
a device being plugged in and one which is spotaneously unplugging.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>