pci: set PCI multi-function bit appropriately.

Set PCI multi-function bit according to multifunction property.
PCI address, devfn ,is exported to users as addr property,
so users can populate pci function(PCIDevice in qemu)
at arbitrary devfn.
It means each function(PCIDevice) don't know whether pci device
(PCIDevice[8]) is multi function or not.
So this patch allows user to set multifunction bit via property
and checks whether multifunction bit is set correctly.

Cc:  Juan Quintela <quintela@redhat.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
