Comment 1 for bug 553737

As it turns out, https://bugzilla.redhat.com/show_bug.cgi?id=578975 is not a bug, but it is a functional regression. From the upstream bug:

09:07 < aliguori> jdstrand, you're adding a scsi disk to an existing scsi
                  adapter
09:07 < aliguori> lspci isn't going to change
09:07 < aliguori> but you won't see anything in /proc/partitions until you
                  rescan the scsi bus
09:07 < aliguori> which is not something that happens automatically
09:07 < aliguori> it's *not* pci hot add
09:08 < aliguori> it's scsi hot add
09:08 < aliguori> http://kbase.redhat.com/faq/docs/DOC-3942
09:08 < aliguori> this is different than what older libvirts did
09:08 < aliguori> older libvirt added a new scsi controller for every disk add
09:08 < aliguori> which was incorrect
09:14 < jdstrand> aliguori: I'm just used to seeing this show up in the guest,
                  so the new behavior, while I understand it, needs to be dealt
                  with in applications that depended on the previously faulty
                  behavior
09:14 < aliguori> yup

So, after using attach-device, I perform the following:
echo "- - -" > /sys/class/scsi_host/host0/scan

Then the disk shows up in the disk.