update leaf nodes instead of removing / readding them

update leaf nodes instead of removing / readding them

if the selected node has its status changed between stop &
running, the node was removed and then readded
during the remove / add process the 'selected' status of the node
was lost if it has one

instead of deleting / readding the node, we update now its content
this was the default behaviour for server nodes, but not for leaf nodes

this fixes a graphical bug where starting/stopping a VM/container would lose
the current selection in the ressource tree