Add last IOSv 15.7.3 version to registry:

- Run check.py and no errors were reported
  - Include the latest version on top of available images
  - Ensure no version/filename duplicates are present
  - Add image size and MD5SUM accordingly

appliances/cisco-iosv.gns3a

@@ -31,6 +31,13 @@
             "download_url": "https://sourceforge.net/projects/gns-3/files",
             "direct_download_url": "http://sourceforge.net/projects/gns-3/files/Qemu Appliances/IOSv_startup_config.img/download"
         },
+        {
+            "filename": "vios-adventerprisek9-m.vmdk.SPA.157-3.M3",
+            "version": "15.7(3)M3",
+            "md5sum": "37c148ffa14a82f418a6e9c2b049fafe",
+            "filesize": 134021120,
+            "download_url": "https://virl.mediuscorp.com/my-account/"
+        },
         {
             "filename": "vios-adventerprisek9-m.vmdk.SPA.156-2.T",
             "version": "15.6(2)T",
@@ -54,6 +61,13 @@
         }
     ],
     "versions": [
+        {
+            "name": "15.7(3)M3",
+            "images": {
+                "hda_disk_image": "vios-adventerprisek9-m.vmdk.SPA.157-3.M3",
+                "hdb_disk_image": "IOSv_startup_config.img"
+            }
+        },
         {
             "name": "15.6(2)T",
             "images": {