block: make 'stats-interval' an array of ints instead of a string

This is the natural JSON representation and prevents us from having to
decode the list manually.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 0e3da8fa206f4ab534ae3ce6086e75fe84f1557e.1447665472.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
