x86/PCI: VMD: Use x86_vector_domain as parent domain

Otherwise APIC code assumes VMD's IRQ domain can be managed by the APIC,
resulting in an invalid cast of irq_data during irq_force_complete_move().

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>