x86/PCI: VMD: Eliminate vmd_vector member from list type

Eliminate unused vmd and vector members from vmd_irq_list and discover the
vector using pci_irq_vector().

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