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>
XOP-385: Up timout in shutdown.py to 600 from 60 seconds.

We only hit this timeout if the VMs fail to cleanly shutdown.
This will be fixed in a different way in trunk where we have the
'VM.shutdown' API call.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #1342 from jonludlam/xop-385

XOP-385