Check for active PCI devices when doing nodedevice operations.

Check for active PCI devices when doing nodedevice operations.

In the current libvirt PCI code, there is no checking whether
a PCI device is in use by a guest when doing node device
detach or reattach.  This causes problems when a device is
assigned to a guest, and the administrator starts issuing
nodedevice commands.  Make it so that we check the list
of active devices when trying to detach/reattach, and only
allow the operation if the device is not assigned to a guest.

Signed-off-by: Chris Lalancette <clalance@redhat.com>