bus/pci: fix VFIO mode

bus/pci: fix VFIO mode

Revert back to using VFIO_PRESENT as a marker to enable compilation
of VFIO-related segments.

VFIO_PRESENT is the combination of user configuration RTE_EAL_VFIO and
kernel version support check.

eal_vfio.h VFIO_PRESENT related check ordered to be compatible with
rte_vfio.h one, no functional modification.

Fixes: 279b581c897d ("vfio: expose functions")

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>