Fix detaching pci device failed

after booting an instance with flavor has pci device,
and then resize the instance, it will raise an "AttributeError".
this because in the method _detach_pci_devices(),
guest_config.devices will have more device not only pci, like disk.

Closes-bug: #1318891

Change-Id: Id52883a4f1fa7d05d61b31674acd1ca57918c3e1
Merge "Fix detaching pci device failed"