It can't be done with virt-manager. That one took me a while to figure out. As far as I can tell, virt-manager works with storage pools. You can make a disk into a storage pool, but you can't add an existing disk directly to a VM.

Luckily the fix is easy: you add the disk to the domain's xml config file by hand. So open up /etc/libvirt/qemu/${YOUR_VM}.xml in your favourite editor and add a <disk> section to the <devices> section:

<disk type='block' device='disk'>
  <driver name='qemu' type='raw' />
  <source dev='/dev/md/storage' />
  <target dev='vdb' bus='virtio' />
</disk>

This will make the host's /dev/md/storage available in the guest as /dev/vdb. After changing a domain's config by hand, you have to reload the config by hand. Log in to your host and issue this command:

# virsh define /etc/libvirt/qemu/jetsetetser.xml
Domain jetser defined from /etc/libvirt/qemu/jetsetetser.xml

Here is a nice resource with more information about managing vms with libvirt.