vhost: rename vhost-backend.c to vhost-kernel.c

This file is totally about vhost-kernel implementation, not generic
code. Let's finally give it corresponding name.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260420202032.714884-4-vsementsov@yandex-team.ru>

hw/virtio/meson.build

@@ -14,7 +14,7 @@ system_virtio_ss.add(files('virtio-qmp.c'))
 
 if have_vhost
   system_virtio_ss.add(files('vhost.c'))
-  system_virtio_ss.add(files('vhost-backend.c', 'vhost-iova-tree.c'))
+  system_virtio_ss.add(files('vhost-kernel.c', 'vhost-iova-tree.c'))
   if have_vhost_user
     system_virtio_ss.add(files('vhost-user.c'))
     system_virtio_ss.add(files('vhost-user-base.c'))

hw/virtio/vhost-kernel.c

@@ -1,5 +1,5 @@
 /*
- * vhost-backend
+ * vhost-kernel
  *
  * Copyright (c) 2013 Virtual Open Systems Sarl.
  *