PCI/portdrv: Allocate MSI/MSI-X vector for Downstream Port Containment

Currently pcie_port_enable_irq_vec() only allocates MSI/MSI-X vectors for
PME, hotplug, and AER.

The Downstream Port Containment feature also supports MSI/MSI-X interrupts,
so allocate a vector for it, too.

Signed-off-by: Liudongdong <liudongdong3@huawei.com>
Signed-off-by: Gabriele Paoloni <gabriele.paoloni@huawei.com>
[bhelgaas: changelog, comment]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>