Recognize this field for VMS_ARRAY typed vmsd fields, then we can do proper
size matching with previous patch.

Note that this is compatible with old -dump-vmstate output, because when
"num" is not there we'll still use the old "size" only.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>