Improve devicemapper driver-status output

Do not print "Data file" and "Metadata file" if they're
not used, and sort/group output.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/devmapper/driver.go

@@ -73,8 +73,23 @@ func (d *Driver) Status() [][2]string {
 		{"Pool Blocksize", units.HumanSize(float64(s.SectorSize))},
 		{"Base Device Size", units.HumanSize(float64(s.BaseDeviceSize))},
 		{"Backing Filesystem", s.BaseDeviceFS},
-		{"Data file", s.DataFile},
-		{"Metadata file", s.MetadataFile},
+		{"Udev Sync Supported", fmt.Sprintf("%v", s.UdevSyncSupported)},
+	}
+
+	if len(s.DataFile) > 0 {
+		status = append(status, [2]string{"Data file", s.DataFile})
+	}
+	if len(s.MetadataFile) > 0 {
+		status = append(status, [2]string{"Metadata file", s.MetadataFile})
+	}
+	if len(s.DataLoopback) > 0 {
+		status = append(status, [2]string{"Data loop file", s.DataLoopback})
+	}
+	if len(s.MetadataLoopback) > 0 {
+		status = append(status, [2]string{"Metadata loop file", s.MetadataLoopback})
+	}
+
+	status = append(status, [][2]string{
 		{"Data Space Used", units.HumanSize(float64(s.Data.Used))},
 		{"Data Space Total", units.HumanSize(float64(s.Data.Total))},
 		{"Data Space Available", units.HumanSize(float64(s.Data.Available))},
@@ -82,17 +97,11 @@ func (d *Driver) Status() [][2]string {
 		{"Metadata Space Total", units.HumanSize(float64(s.Metadata.Total))},
 		{"Metadata Space Available", units.HumanSize(float64(s.Metadata.Available))},
 		{"Thin Pool Minimum Free Space", units.HumanSize(float64(s.MinFreeSpace))},
-		{"Udev Sync Supported", fmt.Sprintf("%v", s.UdevSyncSupported)},
 		{"Deferred Removal Enabled", fmt.Sprintf("%v", s.DeferredRemoveEnabled)},
 		{"Deferred Deletion Enabled", fmt.Sprintf("%v", s.DeferredDeleteEnabled)},
 		{"Deferred Deleted Device Count", fmt.Sprintf("%v", s.DeferredDeletedDeviceCount)},
-	}
-	if len(s.DataLoopback) > 0 {
-		status = append(status, [2]string{"Data loop file", s.DataLoopback})
-	}
-	if len(s.MetadataLoopback) > 0 {
-		status = append(status, [2]string{"Metadata loop file", s.MetadataLoopback})
-	}
+	}...)
+
 	if vStr, err := devicemapper.GetLibraryVersion(); err == nil {
 		status = append(status, [2]string{"Library Version", vStr})
 	}