microvm host is stable (for now)

- per-system default.nix now inherits the source tree via `src` (this
allows modules to be optionally imported from `modules/opt/`)
- adds a default config for microvm hosts
  - enables systemd-networkd by default
  - allows passing in vms
  - binds vm nix-store to host nix-store
  - allows merging systemd jounals
- adds microvms to palatine-hill

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

systems/palatine-hill/default.nix

@@ -1,4 +1,4 @@
-{ inputs, ... }:
+{ inputs, src, ... }:
 {
   users = [
     "alice"
@@ -6,6 +6,6 @@
   ];
   modules = [
     inputs.attic.nixosModules.atticd
-    inputs.microvm.nixosModules.host
+    (src + "/modules/opt/microvm-host.nix")
   ];
 }