XL/objects: Initialize format.json outside of erasure. (#1640)

Fixes #1636

New format now generates a UUID and includes it along with
the order of disks. So that UUID is the real order of disks
and on command line user is able to specify disks in any order.

This pre-dominantly solves our dilemma.
```
{
   "format" : "xl",
   "xl" : {
      "version" : "1",
      "disk": "00e4cf06-5bf5-4bb5-b885-4b2fff4a7959",
      "jbod" : [
         "00e4cf06-5bf5-4bb5-b885-4b2fff4a7959",
         ....
         "c47d2608-5067-4ed7-b1e4-fb81bdbb549f",
         "a543293e-99f1-4310-b540-1e450878e844",
         "18f97cbe-529a-456a-b6d4-0feacf64534d"
      ]
   },
   "version" : "1"
}
```

posix.go

@@ -114,6 +114,9 @@ func checkDiskFree(diskPath string, minFreeDisk int64) (err error) {
 	}
 	di, err := disk.GetInfo(diskPath)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return errDiskNotFound
+		}
 		return err
 	}
 
@@ -203,6 +206,9 @@ func (s fsStorage) ListVols() (volsInfo []VolInfo, err error) {
 	var diskInfo disk.Info
 	diskInfo, err = disk.GetInfo(s.diskPath)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, errDiskNotFound
+		}
 		return nil, err
 	}
 	volsInfo, err = listVols(s.diskPath)
@@ -242,6 +248,9 @@ func (s fsStorage) StatVol(volume string) (volInfo VolInfo, err error) {
 	var diskInfo disk.Info
 	diskInfo, err = disk.GetInfo(s.diskPath)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return VolInfo{}, errDiskNotFound
+		}
 		return VolInfo{}, err
 	}
 	// As os.Stat() doesn't carry other than ModTime(), use ModTime()