xen PV passthru: assign SR-IOV virtual functions to separate virtual slots

VFs are reported as single-function devices in PCI_HEADER_TYPE, which
causes pci_scan_slot() in the PV domU to skip all VFs beyond #0 in the
pciback-provided slot. Avoid this by assigning each VF to a separate
virtual slot.

Acked-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>