CentOS virt - Feb 2010 - Dynamically add virtio storage to guests via libvirt and KVM?

Is this actually possible with KVM on Centos 5.4? I'd like to be able to
grow the storage of a virtualised database server as required.

Both the base server and virtual guest are running Centos 5.4

I've tried adding the storage but it doesn't appear until reboot. I
can't force a scsi device scan as there are no entries under 
/sys/class/scsi_*

Any ideas?

Steve