libxl: add support for rbd qdisk
    
    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: Jim Fehlig <jfehlig@suse.com>