Add missing profile=kvm option to Documentation/kernel-parameters.txt

Whilst looking up what profile=sleep did, I noticed that we missed
adding docs for the most recent addition to the profiler.

Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Adrian Bunk <bunk@kernel.org>