[libvirt] 'attach-interface' failure and RHEL6 qemu-kvm

[Neil Wilson <neil brightbox co uk>]

I've run up against a small issue with the 'attach-interface' facility
in later versions of libvirt.

It doesn't appear to work with the vanilla qemu-kvm package in RHEL6
(which reports as version 0.12.1).

For both attach-interface and attach-device I'm getting the same error.

virsh attach-interface srv-vvh2o network vir0001
error: Failed to attach interface
error: internal error unable to add host net: unknown command:
'host_net_add'

virsh # attach-device srv-vvh2o fred.xml
error: Failed to attach device from fred.xml
error: internal error unable to add host net: unknown command:
'host_net_add'

Looking at qemu monitor the 'qemu-kvm' is indeed short of the
'host_net_add' command and it all seems to be down to an entry in
'qemu_capabilities.c' which switches off 'netdev' for qemu versions less
than 13.

Is there a reason that the RHEL6 version - which seems to have
functioning -netdev parameters and 'netdev_add' qemu commands - has been
excluded?

If its a RHEL6 special patched version is there a way to special case
detect it in the code?

Thanks

Neil