make ssh host keys persistent for VMs

closes #49

nixos/virtual.nix

@@ -42,14 +42,33 @@
   config = {
     system.stateVersion = hypervisorConfig.system.stateVersion;
 
-    microvm = {
+    services.openssh = {
-      shares = [{
+      hostKeys = [{
-        source = "/nix/store";
+        path = "/etc/ssh/host_keys/ssh_host_ed25519_key";
-        mountPoint = "/nix/.ro-store";
+        type = "ed25519";
-        tag = "ro-store";
-        proto = "virtiofs";
       }];
 
+      extraConfig = ''
+        HostKey /etc/ssh/host_keys/ssh_host_ed25519_key
+      '';
+    };
+
+    microvm = {
+      shares = [
+        {
+          source = "/nix/store";
+          mountPoint = "/nix/.ro-store";
+          tag = "ro-store";
+          proto = "virtiofs";
+        }
+        {
+          source = "/var/lib/microvms/${config.networking.hostName}/shares/host_keys";
+          mountPoint = "/etc/ssh/host_keys";
+          tag = "host_keys";
+          proto = "virtiofs";
+        }
+      ];
+
       interfaces = [{
         type = "tap";
         id = "vm-${config.networking.hostName}";