make ssh host keys persistent for VMs

closes #49

nixos/virtual.nix

@@ -42,13 +42,32 @@
   config = {
     system.stateVersion = hypervisorConfig.system.stateVersion;
 
+    services.openssh = {
+      hostKeys = [{
+        path = "/etc/ssh/host_keys/ssh_host_ed25519_key";
+        type = "ed25519";
+      }];
+
+      extraConfig = ''
+        HostKey /etc/ssh/host_keys/ssh_host_ed25519_key
+      '';
+    };
+
     microvm = {
-      shares = [{
+      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";