eal: load only shared libs from driver plugin directories

eal: load only shared libs from driver plugin directories

When we pass a "-d" flag to EAL pointing to a directory, we attempt to load
all files in that directory as driver plugins, irrespective of file type.
This procludes using e.g. the build/drivers directory, as a driver source
since it contains static libs and other files as well as the shared
objects.

By filtering out any files whose filename does not end in ".so", we can
improve usability by allowing other non-driver files to be present in the
driver directory.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>