hw/virtio: Fix brace Werror with clang 6.0.0

The warning is

hw/virtio/vhost-user.c:1319:26: error: suggest braces
      around initialization of subobject [-Werror,-Wmissing-braces]
    VhostUserMsg msg = { 0 };
                         ^
                         {}

While the original code is correct, and technically exactly correct
as per ISO C89, both GCC and Clang support plain empty set of braces
as an extension.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>