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/microvms.nix

@@ -4,7 +4,6 @@
   pkgs,
   ...
 }:
-
 {
-  rad-dev.microvm-host.enable = true;
+  # rad-dev.microvm-host.enable = true;
 }