Never locked out again from my FreeBSD virtual machine for having forget the root password.

The fix:

1. Boot in single user mode
2. Remount the / file system in read and write mode with mount -u / and then mount -a
3. Setup the new password by passwd
4. Boot in multi-user mode with exit
5. ???
6. Profit!

Pheww.

I needed to save this tip somewhere because I know I will forget root password again.