Fix a bug in instance startup with custom hvparams

Since the introduction of OS-specific hvparams, we shouldn't ever use
objects.FillDict directly for instances, but instead go via the cluster
object. Otherwise the os_hvp will be ignored.

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