Report one error less when getting net dev speed
    
    virFileReadAll already logs an error. If reading the 'speed' file
    fails with EINVAL, we log an error even though we ignore it. If it
    fails with other errors, we log two errors.
    
    Use virFileReadAllQuiet - ignore EINVAL and report just one error
    in other cases.
    
    Fixes this error on libvirtd startup:
    2014-06-30 12:47:14.583+0000: 20971: error : virFileReadAll:1297 :
    Failed to read file '/sys/class/net/wlan0/speed': Invalid argument