software/kvm: fix json schemas

string.format must be a string according to
https://json-schema.org/understanding-json-schema/reference/string.html#format

I guess the previous validator was wrong.

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

@@ -368,10 +368,16 @@
               "title": "NBD hostname or IP",
               "description": "hostname (or IP) of the NBD server containing the boot image.",
               "type": "string",
-              "format": [
-                "host-name",
-                "ip-address",
-                "ipv6"
+              "oneOf": [
+                {
+                  "format": "hostname"
+                },
+                {
+                  "format": "ipv4"
+                },
+                {
+                  "format": "ipv6"
+                }
               ],
               "default": "debian.nbd.vifib.net"
             },

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

@@ -172,10 +172,16 @@
       "title": "NBD hostname",
       "description": "hostname (or IP) of the NBD server containing the boot image.",
       "type": "string",
-      "format": [
-        "host-name",
-        "ip-address",
-        "ipv6"
+      "oneOf": [
+        {
+          "format": "hostname"
+        },
+        {
+          "format": "ipv4"
+        },
+        {
+          "format": "ipv6"
+        }
       ],
       "default": "debian.nbd.vifib.net"
     },
@@ -191,10 +197,16 @@
       "title": "Second NBD hostname",
       "description": "hostname (or IP) of the second NBD server (containing drivers for example).",
       "type": "string",
-      "format": [
-        "host-name",
-        "ip-address",
-        "ipv6"
+      "oneOf": [
+        {
+          "format": "hostname"
+        },
+        {
+          "format": "ipv4"
+        },
+        {
+          "format": "ipv6"
+        }
       ]
     },
     "nbd2-port": {