lxd-agent: Fix filesystem metrics
lxd-agent: Fix filesystem metrics

This fixes an issue where available bytes and free bytes would always be
0.

This fixes #10994.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

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