pci least feature matching allocation

in current pci pass-through pci device request via pci stats and claim the device by
another piece of code: pci manager. here is a use case to show why least feature device
match allocation is needed.

suppose we had 2 pci acceleration card, on host1 :
        pci1:  Support neutron physical network 'phy1"
        pci2: same as pci1 but do not support network 'phy1"

and second one on host2 support :
      same as pci1.

now user want a nic card say pci1. in current system scheduler pick host by mach the pci stats, there is no different for host1&host2, no difference for pci1 and pci in the host1, there is 2 problem:

1. pci filter better to pick up host2, if use host1, later user want more features will lack of  pci network for 'phy1', and pick up phy1 might just bad choice because it's might connect to a network usr don't know.

2. if host2 run out device, in host1, should pick up the pci2. this can achieved : pick up least featured card first.