PCI/portdrv: Support multiple interrupts for MSI as well as MSI-X

Root Ports can generate several different interrupts using either MSI or
MSI-X, but we only support that for MSI-X.  Ports that support MSI but not
MSI-X are currently limited to sharing a single interrupt.

Rename pcie_port_enable_msix() to pcie_port_enable_irq_vec() and extend it
to support multiple interrupts using either MSI-X (preferred) or MSI.

Signed-off-by: Gabriele Paoloni <gabriele.paoloni@huawei.com>
[bhelgaas: changelog, reword comments, simplify PME/hotplug no-MSI logic]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>