Here is the patch, that implements the following behaviour

* interface name inside container is automatically generated and equals ethN,
   where N is the number of that interface within current domain
* mac address of that interface inside container is generated automatically by
   function openvzGenerateMac
* if <target dev=""> is specified, use it; otherwise, use the default openvz name,
   i.e., vethN.M for interface ethM in container with veid N
* if <mac address='...'> specified, use it; otherwise, vzctl will generate it automatically
* <target dev> and <mac address> are (re)stored (from)to $veid.conf

the functions openvzGenerateMac and
openvzGenerateVethName are taken from vzctl sources and slightly changed then