dumpStorageTable to handle images with multiple leaf volumes

dumpStorageTable encounters a KeyError exception if an image has
multiple volume chains (and thus multiple leaf volumes) which occurs
when previewing a snapshot.

This patch allows dumpStorageTable to work with images that have
multiple chains / leaf volumes.

Change-Id: Ifa932a47a91caa1a8284ea58678367ca77ed2d9a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=890983
Signed-off-by: Mark Huth <mhuth@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12703
Tested-by: oVirt Jenkins CI Server
Reviewed-by: Ayal Baron <abaron@redhat.com>