eal: pick IOVA as PA if IOMMU is not available

When IOMMU is not available, /sys/kernel/iommu_groups will not be
populated. This is happening since at least 3.6 when VFIO support
was added. If the directory is empty, EAL should not pick IOVA as
VA as the default IOVA mode.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>
Reviewed-by: Jerin Jacob <jerinj@marvell.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>