@Krishna

I was reading up on Virtual Box and ran across this material
concerning some magic key combos that are disallowed in
some virtualization situations.

I thought this might suggest one reason some combos are
not available on some of the later distros.
I miss control-alt-backspace and control-alt-dele when they
aren't available.

See for example,

http://www.virtualbox.org/manual/ch01.html#idp7681328

Cheers