Node: add and export a 'used disk' function
Node: add and export a 'used disk' function

This is similar to iMem.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Balazs Lecz <024c580adbe3324c61e798a30c60cf94ec02e89f@google.com>

leshchevds/ganeti,apyrgio/ganeti,apyrgio/snf-ganeti,andir/ganeti,yiannist/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,mbakke/ganeti,leshchevds/ganeti,bitemyapp/ganeti,onponomarev/ganeti,bitemyapp/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,ganeti/ganeti,apyrgio/ganeti,dimara/ganeti,grnet/snf-ganeti,leshchevds/ganeti,yiannist/ganeti,dimara/ganeti,andir/ganeti,onponomarev/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,ganeti/ganeti