libvirt: use qdisk instead of blktap for Xen

[Alvaro Lopez]

Using libvirt and Xen, nova is using "tap" (Xen 4.0) and "tap2" (Xen > 4.0) for the disk driver.

According to the Xen documentation and ML, the usage of qdisk is preferred
against blktap2 [1].

[1] http://lists.xen.org/archives/html/xen-devel/2013-08/msg02633.html

Moreover, libxenlight (xl, libxl) is the replacement of xend (xm) from Xen >=
4.2. According to the disk configuration documentation for xl [2], the device
driver "(...) should not be specified, in which case libxl will automatically
 determine the most suitable backend."

[2] http://xenbits.xen.org/docs/unstable/misc/xl-disk-configuration.txt

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/113811

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/113811
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=e233998dbfe86339726c3dab2110fc1c4803197f
Submitter: Jenkins
Branch: master

commit e233998dbfe86339726c3dab2110fc1c4803197f
Author: Alvaro Lopez Garcia <email address hidden>
Date: Wed Aug 13 09:57:46 2014 +0200

    libvirt: use qemu (qdisk) disk driver for Xen >= 4.2.0

    According to the Xen ML [1] "qdisk is better supported and blktap is
    unmaintained, we choose qdisk over blktap whenever possible" (change was
    introduced in Xen's commit 0166217 [2]). The Xen documentation states
    that the disk driver "should not be specified, in which case libxl will
    automatically determine the most suitable backend" [3]. However, even if
    libvirt does it OK [4] not setting the driver name so that libxl chooses
    the best backend, it sets the file format to 'raw' so that a wrong
    backend will be chosen by libxl. Since 'qdisk' seems to be the
    preferred option, it seems safe to set it as the backend.

    [1] http://lists.xen.org/archives/html/xen-devel/2013-08/msg02633.html
    [2] http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=0166217103e18368424fbd5ffff01c1ea50d0b17
    [3] http://xenbits.xen.org/docs/unstable/misc/xl-disk-configuration.txt
    [4] http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/libxl/libxl_conf.c;h=1210500858b00b1d10b0bc921ab8b5efc87e630f;hb=HEAD#l847

    Closes-Bug: #1326238
    Change-Id: Ia52d52a01916fac92dc8e648c462e237c0bfc915