x86/PCI: VMD: Convert to use pci_alloc_irq_vectors() API

Convert to use the pci_alloc_irq_vectors() API.

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