Hello,
I know this post is old, but it was my old problem, and I was able to solve it today with the help of this reference:
https://forums.virtualbox.org/viewtopic.php?t=15679
I followed and it worked, and finally I am enjoying an LXDE VM Guest with seamless mode enabled, I am very happy

Just if some one have same problem.
Best regards
joejac
My Guest VM is: PCLinuxOS 2011 Linux 2.6.38.8-pclos3.bfs kernel