x86/PCI: VMD: Allocate IRQ lists with correct MSI-X count

To reduce the amount of memory required for IRQ lists, only allocate their
space after calling pci_msix_enable_range() which may reduce the number of
MSI-X vectors allocated.

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>