tests/qtest/vhost-user-blk-test: use memory-backend-shm

`memory-backend-memfd` is available only on Linux while the new
`memory-backend-shm` can be used on any POSIX-compliant operating
system. Let's use it so we can run the test in multiple environments.

Since we are here, let`s remove `share=on` which is the default for shm
(and also for memfd).

Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20240618100527.145883-1-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tests/qtest/vhost-user-blk-test.c

@@ -906,7 +906,7 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
                            vhost_user_blk_bin);
 
     g_string_append_printf(cmd_line,
-            " -object memory-backend-memfd,id=mem,size=256M,share=on "
+            " -object memory-backend-shm,id=mem,size=256M "
             " -M memory-backend=mem -m 256M ");
 
     for (i = 0; i < vus_instances; i++) {