PCI: Provide sensible IRQ vector alloc/free routines

Add a function to allocate and free a range of interrupt vectors, using
MSI-X, MSI or legacy vectors (in that order) based on the capabilities of
the underlying device and PCIe complex.

Additionally a new helper is provided to get the Linux IRQ number for given
device-relative vector so that the drivers don't need to allocate their own
arrays to keep track of the vectors for the multi vector MSI-X case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alexander Gordeev <agordeev@redhat.com>