<interface type='bridge'>
     <mac address='00:16:3e:34:21:9e'/>
     <source bridge='eth10'/>
   </interface>
   <interface type='ethernet'>
     <ip address="192.168.122.3" />
   </interface>
instead.

Second. If I understood it right, the following description
   <interface type='bridge'>
     <mac address='00:16:3e:34:21:9e'/>
     <source bridge='eth10'/>
   </interface>
will result in the (approximataly) following:
     # vzctl --netiff_add eth10,00:16:3e:34:21:9e
This behaviour contradicts with description found in docs (in which
<source> tag specify interface in host, not in container). I think,
the previous bridge must be specified as
   <interface type='bridge'>
     <mac address='00:16:3e:34:21:9e'/>
-     <source bridge='eth10'/>
+     <target dev='eth10'/>
   </interface>