vfio-pci: Use cached MSI/MSI-X capabilities

We now cache the MSI/MSI-X capability offsets in the struct pci_dev,
so no need to find the capabilities again.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>