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
tests: update _get_instance_xml to accept custom flavor object

Currently LibvirtVifTestCase._get_instance_xml creates a default
flavor object and uses it for all tests. This makes it dificult
to test settings that are being activated by values in extra
spects.

Allowing the method to accept a flavor object and use the default
one, if not provided.

Change-Id: I3835aa5e6cf09268b132b9de377004c0e8df9cb0
libvirt: Set guest machine type on system z

The domain XML for guests on system z must have the machine type
"s390-ccw-virtio" to set up the device addresses properly. The
default value "s390-virtio" (without "ccw") which is listed by
libvirts get_host_capabilities call cannot be used properly for
guests.

Partial-Implements blueprint libvirt-kvm-systemz

Change-Id: Ib9cfb0ad8db3fe9bfc6b1302761f728fe56c0602
Imported Translations from Transifex

For more information about this automatic import see:
https://wiki.openstack.org/wiki/Translations/Infrastructure

Change-Id: I2c2f678a9d5b1ec662dbeb9672904553bdd2c5c5
Merge "libvirt: Set guest machine type on system z"
Merge "libvirt: use qemu (qdisk) disk driver for Xen >= 4.2.0"
Merge "tests: update _get_instance_xml to accept custom flavor object"
Merge "Imported Translations from Transifex"