ivshmem: improve error handling The test whether the chardev is an AF_UNIX socket rejects "-chardev socket,id=chr0,path=/tmp/foo,server,nowait -device ivshmem,chardev=chr0", but fails to explain why. Use an explicit error on why a chardev may be rejected. Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com> Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>