xen/pci: introduce PF<->VF links

xen/pci: introduce PF<->VF links

Add links between a VF's struct pci_dev and its associated PF struct
pci_dev.

The hardware domain is expected to remove the associated VFs before
removing the PF. If removal happens out of order, print a warning and
return an error. This means that VFs can only exist with an associated
PF.

Additionally, if the hardware domain attempts to remove a PF with VFs
still present, mark the PF and VFs broken, because Linux Dom0 has been
observed to not respect the error returned.

Move the calls to pci_get_pdev() and pci_add_device() down to avoid
dropping and re-acquiring the pcidevs_lock().

Check !pdev->pf_pdev before adding the VF to the list to guard against
adding it multiple times.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>