CA-109035: Remove the correct xenstore key when PCI unplugging.

We were always attempting to remove dev-0 instead of dev-n. This
led to multiple attempts to e.g. deassign_device for all but the
first passed through device. It also led to a cancelled shutdown
attempting to unplug everything again without any exception handler,
leading to an infinite loop.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>