kvm: Relax external-disk-size minimum

That's nothing wrong with such small disk, and the default seems reasonable
enough.

software/kvm/instance-kvm-input-schema.json

@@ -204,7 +204,7 @@
       "title": "Size of additional disk to create for virtual machine, in Gigabytes",
       "description": "Specify the size of additional disk to create for virtual machine in data folder of SlapOS Node. Requires instance_storage_home to be configured on SlapOS Node.",
       "type": "integer",
-      "minimum": 10,
+      "minimum": 1,
       "default": 20
     },
     "external-disk-format": {