grafana: use available bytes when computing rootfs used space
grafana: use available bytes when computing rootfs used space

lxd_filesystem_avail_bytes is <= lxd_filesystem_free_bytes but the
former represents the amount that is directly usable. As such, use
it to deduce the used space.

Signed-off-by: Simon Deziel <eca3f1a14b730e70ce4898287e4fc2f9966fa3a6@canonical.com>

stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd