xl: StatVol and ListVols should handle cases when disks are missing. (#1703)
xl: StatVol and ListVols should handle cases when disks are missing. (#1703)

Remove unnecessary code, handle cases of missing disk.
hackintoshrao/minio,abperiasamy/minio,aead/minio,harshavardhana/minio,harshavardhana/minio,dutchcoders/minio,fwessels/minio,abperiasamy/minio,krishnasrinivas/minio,rushenn/minio,luomeiqin/minio,fwessels/minio,aead/minio,krishnasrinivas/minio-daily,krishnasrinivas/minio,harshavardhana/minio,aead/minio,krisis/minio,jimjag/minio,koolhead17/minio,brendanashworth/minio,abperiasamy/minio,krishnasrinivas/minio,vadmeste/minio,dutchcoders/minio,koolhead17/minio,balamurugana/minio,harshavardhana/minio,harshavardhana/minio,alexellis/minio,krisis/minio,brendanashworth/minio,luomeiqin/minio,rushenn/minio,rushenn/minio,jimjag/minio,fwessels/minio,NitishT/minio,dutchcoders/minio,dutchcoders/minio,mf-00/newgo,jimjag/minio,donatello/minio,mkash32/minio,hackintoshrao/minio,hackintoshrao/minio,minio/minio,abperiasamy/minio,dvstate/minio,dvstate/minio,hackintoshrao/minio,alexellis/minio,donatello/minio,dvstate/minio,brendanashworth/minio,dvstate/minio,rushenn/minio,mf-00/newgo,harshavardhana/minio,luomeiqin/minio,vadmeste/minio,harshavardhana/minio,aead/minio,jimjag/minio,aead/minio,krishnasrinivas/minio,aead/minio,NitishT/minio,fwessels/minio,minio/minio,aead/minio,mkash32/minio,harshavardhana/minio,NitishT/minio,brendanashworth/minio,NitishT/minio,krishnasrinivas/minio-daily,balamurugana/minio,aead/minio,luomeiqin/minio