nfs_apply :: exportfs sometimes leaves exports behind

On my machine i had a case where /etc/exports was updated but the old
exports were still there. This was leading to
"exportfs: duplicated export entries" and eventually leading to nfs
being not available for the box.

changing the command exportfs -r to exportfs -ar seems to address this
issue.

Signed-off-by: BlackEagle <ike.devolder@gmail.com>
Merge pull request #2851 from BlackIkeEagle/reload-nfs-exports

guests/linux: use exportfs -a flag to avoid stale exports