iommu/vt-d: Cure VF irqdomain hickup

The recent changes to store the MSI irqdomain pointer in struct device
missed that Intel DMAR does not register virtual function devices.  Due to
that a VF device gets the plain PCI-MSI domain assigned and then issues
compat MSI messages which get caught by the interrupt remapping unit.

Cure that by inheriting the irq domain from the physical function
device.

Ideally the irqdomain would be associated to the bus, but DMAR can have
multiple units and therefore irqdomains on a single bus. The VF 'bus' could
of course inherit the domain from the PF, but that'd be yet another x86
oddity.

Fixes: 85a8dfc57a0b ("iommm/vt-d: Store irq domain in struct device")
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Link: https://lore.kernel.org/r/draft-87eekymlpz.fsf@nanos.tec.linutronix.de

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