xl: pci multi-function passthrough v2

xl: pci multi-function passthrough v2

Implement PCI pass-through for multi-function devices. The supported BDF
notation is: BB:DD.* - therefore passing-through a subset of functions or
remapping the function numbers is not supported except for when passing
through a single function which will be a virtual function 0.

Letting qemu automatically select the virtual slot is not supported in
multi-function passthrough since the qemu-dm protocol can not actually
handle that case.

Also fix format error in xl pci-list-assignable.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>