cmdlib.py: don't use GetHypervisor

In cmdlib we must only use the hypervisor class, and never instantiate
it. As such we have to call GetHypervisorClass instead, to avoid
getting an instance of it.

This fixes Issue 316, because __init__ is not called from masterd
anymore, and thus can't fail on EnsureDirs.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>