PCI/MSI: Fix MSI IRQ domains for VFs on virtual buses

SR-IOV creates a virtual bus where bus->self is NULL.  When we add VFs and
scan for an MSI domain, pci_set_bus_msi_domain() dereferences bus->self,
which causes a kernel NULL pointer dereference oops.

Scan up to the parent bus until we find a real bridge where we can get the
MSI domain.

[bhelgaas: changelog]
Fixes: 44aa0c65 ("PCI/MSI: Add hooks to populate the msi_domain field")
Tested-by: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>