eal: avoid error for non-existent default PMD path

eal: avoid error for non-existent default PMD path

If the default location for the PMD .so files does not exist, it should
not be treated as a fatal error condition like an incorrect path on the
command line. Therefore check that the path exists and is a directory
before adding it to the list of paths to check for PMDs.

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