https://help.ubuntu.com/community/ServerFaq#What_are_the_differences_between_the_server_and_virtual_kernels.3F
Type: Posts; User: Slezhuk; Keyword(s):
https://help.ubuntu.com/community/ServerFaq#What_are_the_differences_between_the_server_and_virtual_kernels.3F
It works, thanks!
p.s virsh define /etc/libvirt/qemu/domain.conf works too- no need for rebooting entire libvirt.
In my case there was entry in /etc/apt/sources.list.d/lucid-partner.list