sockets: Fix socket_sockaddr_to_address_unix() for abstract sockets

Commit 776b97d360 "qemu-sockets: add abstract UNIX domain socket
support" neglected to update socket_sockaddr_to_address_unix().  The
function returns a non-abstract socket address for abstract
sockets (wrong) with a null @path (also wrong; a non-optional QAPI str
member must never be null).

The null @path is due to confused code going back all the way to
commit 17c55decec "sockets: add helpers for creating SocketAddress
from a socket".

Add the required special case, and simplify the confused code.

Fixes: 776b97d360
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

util/qemu-sockets.c

@@ -1270,10 +1270,20 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
 
     addr = g_new0(SocketAddress, 1);
     addr->type = SOCKET_ADDRESS_TYPE_UNIX;
-    if (su->sun_path[0]) {
-        addr->u.q_unix.path = g_strndup(su->sun_path, sizeof(su->sun_path));
+#ifdef CONFIG_LINUX
+    if (!su->sun_path[0]) {
+        /* Linux abstract socket */
+        addr->u.q_unix.path = g_strndup(su->sun_path + 1,
+                                        sizeof(su->sun_path) - 1);
+        addr->u.q_unix.has_abstract = true;
+        addr->u.q_unix.abstract = true;
+        addr->u.q_unix.has_tight = true;
+        addr->u.q_unix.tight = salen < sizeof(*su);
+        return addr;
     }
+#endif
 
+    addr->u.q_unix.path = g_strndup(su->sun_path, sizeof(su->sun_path));
     return addr;
 }
 #endif /* WIN32 */