virtualbox.org

in 1.6.4 changelog i see:

# Linux hosts: fixed long guest shutdown time when serial port is enabled

but when i shut down my VM (dos 6.0) i must wait a lot of time...
ok isn't really important (my assembler programs works fine)... but the problem isn't solved!

I think they tested it mostly with current OS systems, not ancient ones like MSDOS. You can fill a bug report for it, see the Forum FAQ for how to do that.