libsystemd-bus: copy over kdbus provided 128-bit bus id

kdbus now copies the bus unique id back to userspace in the hello
ioctl(). Use these bytes to set the server id of the sd_bus.

src/libsystemd-bus/bus-kernel.c

@@ -357,6 +357,9 @@ int bus_kernel_take_fd(sd_bus *b) {
         b->bus_client = true;
         b->can_fds = !!(hello.conn_flags & KDBUS_HELLO_ACCEPT_FD);
 
+        /* the kernel told us the UUID of the underlying bus */
+        memcpy(b->server_id.bytes, hello.id128, sizeof(b->server_id.bytes));
+
         r = bus_start_running(b);
         if (r < 0)
                 return r;

src/libsystemd-bus/kdbus.h

@@ -276,6 +276,8 @@ struct kdbus_cmd_hello {
 	__u64 bloom_size;	/* The bloom filter size chosen by the
 				 * bus owner */
 	__u64 pool_size;	/* maximum size of pool buffer */
+	__u8 id128[16];		/* the unique id of the bus */
+
 	struct kdbus_item items[0];
 };