provide pci_request_irq / pci_free_irq helpers

[Christoph Hellwig]

PCI drivers that support multiple MSI or MSI-X vectors currently have a
lot of boileplate code to generate names for each vector (or simply use
the same name for all of them, which isn't nice either).  This series
adds new helpers that allocate and free a name based on a format string
passed to the request_irq wrapper.

They also hide the [pci_dev,vector] to Linux irq number translation,
in fact the example NVMe driver conversion now never sees the Linux irq
number.  That might be useful to isolate PCI drivers from our IRQ numbers,
although to have a full abstraction we'd need similar wrappers for
{enable,disable,synchronize}_irq as well.