It is a problem in virtualBox and has to do with security. Before 4.1.8 symlinks worked but was seriously flawed. And the discision was made to remove symlink support.
See this comment:
Symbolic link creation from within a guest has been disabled in VirtualBox 4.1.8 for security reasons. A guest could create symbolic links which point outside the assigned host directory. This has nothing to do with any ext3/ext4 bug. And the guest is still able to read symlinks which are created on the host.
Sorry for the late statement.
If you do
VBoxManage setextradata VM_NAME VBoxInternal2/SharedFoldersEnableSymlinksCreate/SHARE_NAME 1
Then your guest will be able to create symlinks again. But for security reasons (see above) this is disabled by default. The fix to prevent dangerous symlinks from the guest is very complicated, therefore we decided to not allow any guest to create any symlink to work around the security problem.
(I took the liberty to fix a bug in the comment ;) )
- and also take note that you need to restart vBox for the change to activate.