vmstate: remove const for put operations

vmstate: remove const for put operations

In a later patch, we introduce pre_save() and post_save() functions.
The whole point of that operation is to change things in the state.
Without this patch, we have to remove the const qualifier in each
use with a cast

Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>