missing block script support for qemu in libxl

[Olaf Hering <olaf@aepfle.de>]

Why does libxl now allow script= with backend=tap|qdisk? See
tools/libxl/libxl_device.c:disk_try_backend.

Ideally the script should prepare the backend storage based on info from
target=. Then the script should report either the dentry to be used by
qemu back to libxl, or it should setup the "physical-device" entry to be
used by blkback.

Right now the target= info is passed to the script, but its also used as
->pdev_path for qemu. Obvisouly qemu cant possibly make sense of the
target= info, because its meant for the script=.

tools/libxl/check-xl-disk-parse has some script examples. I think right
now its not possible to boot an HVM guest from iscsi, for example.

Should the logic in libxl be changed to make scripts= more useful, and
let libxl read info for qemu which has to be provided by the script?

Olaf