plug a i-am-everywhere leak

plug a i-am-everywhere leak
 + delete pimpl_netcard when freeing a host
 + As the netcard of a physical host is shared by VMs running on it,
   it leads to double frees.
 + A solution is to explicity destroy the VMs when they are not needed
   anymore.
If one doesn't like it, feel free to simplify the VM stuff ;)