_ExecuteKVMRuntime: fix hv parameter fun

When executing the kvm runtime we were currently accessing a mix of the
parameters as configured currently on the instance and the ones it was
started with. We were doing it without a precise criteria, but quite by
chance we got it *almost* right. The only remaining issue was that when
ganeti was upgraded and some parameters were added, trying to access
them from the "old" ones caused a keyerror, since they weren't present
back when the instance was started.

To fix this:
  - We fill the startup-time dict with any new parameter
  - We provide a clear guideline on which version of the parameters to
    access, and about the fact that new parameters must have an
    instance-migration backwards compatible default

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