Force vdsm to stay with libvirt <= 1.2.7

Libvirt removed silent support for min_guarantee parameter when
running on Qemu. This prevents us from starting any new VMs
or migrating VMs to hosts with newer versions of libvirt.

See https://bugzilla.redhat.com/show_bug.cgi?id=1122455 for
the libvirt reasoning.

This patch should force hosts to stay with older versions of libvirt
until this is resolved.

Label: ovirt-3.5-only
Change-Id: Ib8e7ba73a5a8f1407b077754fc768653a6e29efc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1138340
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32764
Reviewed-by: Dan Kenigsberg <danken@redhat.com>