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