vfio: avoid enabling while the module is not loaded

vfio: avoid enabling while the module is not loaded

When vfio module is not loaded when kernel support vfio feature,
the routine still try to open the container to get file
description.

This action is not safe, and of course got error messages:

EAL: Detected 40 lcore(s)
EAL:   unsupported IOMMU type!
EAL: VFIO support could not be initialized
EAL: Setting up memory...

This may make user confuse, this patch make it reasonable
and much more smooth to user.

Signed-off-by: Michael Qiu <michael.qiu@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>