- As it turns out, not only MSI-X is broken for devices passed through by
  VMware up to at least ESXi 5.1. Actually, using INTx in that case instead
  may still result in interrupt storms, with MSI being the only working
  option in some configurations. So introduce a PCI_QUIRK_DISABLE_MSIX quirk
  which only blacklists MSI-X but not also MSI and use it for the VMware
  PCI-PCI-bridges. Note that, currently, we still assume that if MSI doesn't
  work, MSI-X won't work either - but that's part of the internal logic and
  not guaranteed as part of the API contract. While at it, add and employ
  a pci_has_quirk() helper.
  Reported and tested by: Paul Bucher
- Use NULL instead of 0 for pointers.

Submitted by:	jhb (mostly)
Approved by:	jhb
MFC after:	3 days
Adjust comments to fit within 80-columns.