xl/libxl already supports qemu's network-based block backends
such as nbd and rbd. libvirt has supported configuring such
<disk>s for long time too. This patch adds support for rbd
disks in the libxl driver by generating a rbd device URL from
the virDomainDiskDef object. The URL is passed to libxl via the
pdev_path field of libxl_device_disk struct. libxl then passes
the URL to qemu for cosumption by the rbd backend.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>