add pci multifunction support

add pci multifunction support

multifunction device should be define without the .function

hostpci0: 00:00

example
-------
if 00:00.0
   00:00.1
   00:00.2

exists,

then we generate the multifunction devices

-device (pci-assign|vfio-pci),host=00:00.0,id=hostpci0.0,bus=...,addr=0x0.0,multifunction=on
-device (pci-assign|vfio-pci),host=00:00.1,id=hostpci0.1,bus=...,addr=0x0.1
-device (pci-assign|vfio-pci),host=00:00.2,id=hostpci0.2,bus=...,addr=0x0.2

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>