xend: passthrough: loosen the pci co-assignment for pv guest

xend: passthrough: loosen the pci co-assignment for pv guest

In current xend, we can not assign co-assignment devices to different
guests, even for pv guests. This patch loosens the policy for pv
guest: if none of the co-assignment devices have been assigned to hvm
guest, we can assign the devices to different pv guests.

The patch also adds a detection: if a device has been assigned to
guest, we can't try to "xm pci-attach" it to the same guest again.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>