porthos: move files into 'machines' directory

But keep 'porthos.nix' at the root of the repository. I feel like it is
cleaner to keep device specific files at the root.

machines/porthos/boot.nix

@@ -0,0 +1,23 @@
+# Boot configuration
+{ ... }:
+
+{
+  boot = {
+    # Use the GRUB 2 boot loader.
+    loader.grub = {
+      enable = true;
+      version = 2;
+      # Define on which hard drive you want to install Grub.
+      device = "/dev/sda";
+    };
+
+    initrd = {
+      availableKernelModules = [ "uhci_hcd" "ahci" "usbhid" ];
+      kernelModules = [ "dm-snapshot" ];
+    };
+
+    kernelModules = [ "kvm-intel" ];
+
+    extraModulePackages = [ ];
+  };
+}