backends: Improve error messages when property can no longer be set

When you try to set virtio-rng property "filename" after the backend
has been completed with user_creatable_complete(), the error message
blames "insufficient permission":

    $ qemu-system-x86_64 -S -display none -nodefaults -monitor stdio -object rng-random,id=rng0 -device virtio-rng,id=vrng0,rng=rng0
    QEMU 7.1.50 monitor - type 'help' for more information
    (qemu) qom-set /objects/rng0 filename /dev/random
    Error: Insufficient permission to perform this operation

This implies it could work with "sufficient permission".  It can't.
Change the error message to:

    Error: Property 'filename' can no longer be set

Same for cryptodev-vhost-user property "chardev", rng-egd property
"chardev", and vhost-user-backend property "chardev".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221012153801.2604340-3-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
[Commit message tidied up]

backends/cryptodev-vhost-user.c

@@ -339,7 +339,7 @@ static void cryptodev_vhost_user_set_chardev(Object *obj,
                       CRYPTODEV_BACKEND_VHOST_USER(obj);
 
     if (s->opened) {
-        error_setg(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, "Property 'chardev' can no longer be set");
     } else {
         g_free(s->chr_name);
         s->chr_name = g_strdup(value);

backends/rng-egd.c

@@ -116,7 +116,7 @@ static void rng_egd_set_chardev(Object *obj, const char *value, Error **errp)
     RngEgd *s = RNG_EGD(b);
 
     if (b->opened) {
-        error_setg(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, "Property 'chardev' can no longer be set");
     } else {
         g_free(s->chr_name);
         s->chr_name = g_strdup(value);

backends/rng-random.c

@@ -96,7 +96,7 @@ static void rng_random_set_filename(Object *obj, const char *filename,
     RngRandom *s = RNG_RANDOM(obj);
 
     if (b->opened) {
-        error_setg(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, "Property 'filename' can no longer be set");
         return;
     }
 

backends/vhost-user.c

@@ -141,7 +141,7 @@ static void set_chardev(Object *obj, const char *value, Error **errp)
     Chardev *chr;
 
     if (b->completed) {
-        error_setg(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, "Property 'chardev' can no longer be set");
         return;
     }