Make NIC VM readOnly (#4444)

Author: EvgenyAgafonchikov

specification/network/resource-manager/Microsoft.Network/stable/2018-08-01/networkInterface.json

@@ -930,7 +930,8 @@
       "properties": {
         "virtualMachine": {
           "$ref": "./network.json#/definitions/SubResource",
-          "description": "The reference of a virtual machine."
+          "description": "The reference of a virtual machine.",
+          "readOnly": true
         },
         "networkSecurityGroup": {
           "$ref": "./networkSecurityGroup.json#/definitions/NetworkSecurityGroup",