machine-image: introduce two different classes of images

This distuingishes two different classes of images, one for the purpose
of npsawn-like containers, i.e. "machines", and one for portable
services.

This distinction is mostly about search paths. We look for machine
images in /var/lib/machines and for portable images in
/var/lib/portables.

src/import/import.c

@@ -75,7 +75,7 @@ static int import_tar(int argc, char *argv[], void *userdata) {
                 }
 
                 if (!arg_force) {
-                        r = image_find(local, NULL);
+                        r = image_find(IMAGE_MACHINE, local, NULL);
                         if (r < 0)
                                 return log_error_errno(r, "Failed to check whether image '%s' exists: %m", local);
                         else if (r > 0) {
@@ -170,7 +170,7 @@ static int import_raw(int argc, char *argv[], void *userdata) {
                 }
 
                 if (!arg_force) {
-                        r = image_find(local, NULL);
+                        r = image_find(IMAGE_MACHINE, local, NULL);
                         if (r < 0)
                                 return log_error_errno(r, "Failed to check whether image '%s' exists: %m", local);
                         else if (r > 0) {