[daemon] Remove absent instances in daemon c'tor

Log and remove, from the db, instances whose images are missing when the
daemon comes back up. This covers cases where the user deletes instances
directly from the backend, or otherwise removes them from disk.

src/daemon/daemon.cpp

@@ -901,6 +901,14 @@ mp::Daemon::Daemon(std::unique_ptr<const DaemonConfig> the_config)
         }
 
         auto vm_image = fetch_image_for(name, config->factory->fetch_type(), *config->vault);
+        if (!QFile::exists(vm_image.image_path))
+        {
+            mpl::log(mpl::Level::warning, category,
+                     fmt::format("Could not find image for '{}'. Expected location: {}", name, vm_image.image_path));
+            invalid_specs.push_back(name);
+            continue;
+        }
+
         const auto instance_dir = mp::utils::base_dir(vm_image.image_path);
         const auto cloud_init_iso = instance_dir.filePath("cloud-init-config.iso");
         mp::VirtualMachineDescription vm_desc{spec.num_cores,