add iface to xen instance

hi
I'm trying to add a virtual interface to eth0 the usual way in a Xen instance
but I'm getting
Failed to save interface :
SIOCGIFADDR: Cannot assign requested address
SIOCSIFBROADCAST: Cannot assign requested address
SIOCSIFFLAGS: Cannot assign requested address

I can add it at the activate on boot section but it doesnt show up at activated now section.
Then I tried it through Cloudmin, but I dont have the choice for virtual interface.

The virtual interface eth0:1 should have the same IP as the eth0