Add “gnt-os modify” for per-os-hypervisor parameters

Introduce “gnt-os modify” command to make it possible to set the
per-os-hypervisor parameters.

Signed-off-by: René Nussbaumer <rn@google.com>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>