xend: pci: improve the assignability checking

xend: pci: improve the assignability checking

1) fix some small typos in util/pci.py;
2) find_all_the_multi_functions(): BDFs of a multi-function PCIe
device could be different in all the 3 fields (bus, device, function),
so we need self.find_parent() and list all t
he BDFs below the parent;
3) to assign a device of the must-be-co-assigned devices, we require
all the related devices should be owned by pciback;
4) detect and disallow duplicate pci string specified in guest config
file due to carelessness.

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