In nova/virt/libvirt/utils.py the method pick_disk_driver_name() returns
"tap" for non-blockdevices on Xen. This leads to an exception on the
xend service. The returned driver name should be "file" instead of
"tap".
fixes bug 1084618
Change-Id: Ib87f18f56afb3c5f34c30e9aea605dad24a36cdf