vmspawn: Use builtin vdagent instead of spicevmc

The builtin one also makes the clipboard and such work. spicevmc
is only required for remote desktop use cases, so let's use the 
builtin one instead.

src/vmspawn/vmspawn.c

@@ -2965,9 +2965,9 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
                         return r;
 
                 r = qemu_config_section(config_file, "chardev", "vdagent",
-                                        "backend", "spicevmc",
-                                        "debug", "0",
-                                        "name", "vdagent");
+                                        "backend", "qemu-vdagent",
+                                        "clipboard", "on",
+                                        "debug", "0");
                 if (r < 0)
                         return r;