Fix for disk speed problem

Issue: paradoxxxzero#489
The original code was accumulating sectors of disks/block devices and partitions, duplicating the read and write sectors counting.
Now the code gets the block device list and just accumulates their sectors.
I found a division of these sectors (512 bytes each) by 1024 and 8 (8192 => 8k) to get MiB, but that's not right, it is needed to divide by 2k (or *512/1024/1024) not 8k, maybe due to the accumulation of block devices and partitions, however, when operating on one partition (dd if=/dev/sda1 of=/dev/null bs=4k), it duplicates the accumulated sectors, so you need to divide by 4k, not 8k, getting half the speed. But if you are operating directly on a disk device (dd if=/dev/sda of=/dev/null bs=4k), does not duplicate the accumulated sectors, so you need to divide by 2k, not 8k, getting half of half the speed.
Nowadays, disk space is in MB, not in MiB, so I adjusted this too.

system-monitor@paradoxxx.zero.gmail.com/extension.js

@@ -1495,7 +1495,7 @@ const Disk = class SystemMonitor_Disk extends ElementBase {
         this.last = [0, 0];
         this.usage = [0, 0];
         this.last_time = 0;
-        this.tip_format(_('MiB/s'));
+        this.tip_format(_('MB/s'));  // changed from MiB to MB (space standard)
         this.update();
     }
     update_mounts(mounts) {
@@ -1509,21 +1509,34 @@ const Disk = class SystemMonitor_Disk extends ElementBase {
             let as_r = source.load_contents_finish(result);
             let lines = parse_bytearray(as_r[1]).toString().split('\n');
 
+            let blocks = Gio.file_new_for_path('/sys/block');
+            let list = blocks.enumerate_children('standard::name', 0, null);
+            let disks = [];
+            let disk = null;
+            while (disk = list.next_file(null)) {
+                disks.push(disk.get_name());
+            }
+
             for (let i = 0; i < lines.length; i++) {
                 let line = lines[i];
                 let entry = line.trim().split(/[\s]+/);
                 if (typeof (entry[1]) === 'undefined') {
                     break;
                 }
-                accum[0] += parseInt(entry[5]);
-                accum[1] += parseInt(entry[9]);
+
+                if (disks.indexOf(entry[2]) != -1) {
+                    // sectors of 512 bytes: / 2048 to get MiB or
+                    //                       * 512 / 1000000 to get MB
+                    accum[0] += parseInt(entry[5]) * 512 / 1000000; // read
+                    accum[1] += parseInt(entry[9]) * 512 / 1000000; // write
+                }
             }
 
-            let time = GLib.get_monotonic_time() / 1000;
-            let delta = (time - this.last_time) / 1000;
+            let time = GLib.get_monotonic_time() / 1000000; // micro to seconds
+            let delta = time - this.last_time;
             if (delta > 0) {
                 for (let i = 0; i < 2; i++) {
-                    this.usage[i] = ((accum[i] - this.last[i]) / delta / 1024 / 8);
+                    this.usage[i] = ((accum[i] - this.last[i]) / delta);
                     this.last[i] = accum[i];
                 }
             }
@@ -2312,8 +2325,8 @@ const Gpu = class SystemMonitor_Gpu extends ElementBase {
             this.vals = [0, 0];
             this.tip_vals = [0, 0];
         } else {
-            // we subtract percentage from memory because we do not want memory to be 
-            // "accumulated" in the chart with utilization; these two measures should be 
+            // we subtract percentage from memory because we do not want memory to be
+            // "accumulated" in the chart with utilization; these two measures should be
             // independent
             this.vals = [this.percentage, this.mem / this.total * 100 - this.percentage];
             this.tip_vals = [Math.round(this.vals[0]), this.mem];