builder: normalize architectures

Normalize the target architecture, and also each architecture when
checking for a compatible image.

This sort of reverts the effects of
commit 8864c47b94, but at least it is
possible to build e.g. Debian-based amd64 images on any x86_64 system
without being considered as foreign architecture.

builder/builder.ml

@@ -238,7 +238,7 @@ let main () =
   let item =
     try List.find (
       fun (name, { Index.arch = a }) ->
-        name = arg && arch = a
+        name = arg && arch = normalize_arch a
     ) index
     with Not_found ->
       error (f_"cannot find os-version '%s' with architecture '%s'.\nUse --list to list available guest types.")

builder/cmdline.ml

@@ -295,6 +295,7 @@ read the man page virt-builder(1).
     match arch with
     | "" -> Config.host_cpu
     | arch -> arch in
+  let arch = normalize_arch arch in
 
   (* If user didn't elect any root password, that means we set a random
    * root password.