CA-100489: xenopsd: ignore errors in PCI.unplug when VM does not exist

This makes it consistent with VIF.unplug and VBD.unplug, and prevents xenopsd
from spinning in VM.shutdown with PCI devices present.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1167 from robhoes/ca100489

CA-100489: xenopsd: ignore errors in PCI.unplug when VM does not exist