vfio/pci: use PCI_MSIX_FLAGS on retrieving the MSIX entries

Even PCI_CAP_FLAGS has the same value as PCI_MSIX_FLAGS, the later one is
the more proper on retrieving MSIX entries.

This patch uses PCI_MSIX_FLAGS to retrieve the MSIX entries.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>