Migration to an older libvirt (pre v1.3.0-175-g71408079) is broken
because older versions of libvirt generated different channel paths and
they didn't drop the default paths when parsing domain XMLs. We'd get
such a nice error message:

    internal error: process exited while connecting to monitor:
    2016-07-08T15:28:02.665706Z qemu-kvm: -chardev socket,
    id=charchannel0,path=/var/lib/libvirt/qemu/channel/target/
    domain-3-nest/org.qemu.guest_agent.0,server,nowait: Failed to bind
    socket to /var/lib/libvirt/qemu/channel/target/domain-3-nest/
    org.qemu.guest_agent.0: No such file or directory

That said, we should not even format the default paths when generating a
migratable XML.

https://bugzilla.redhat.com/show_bug.cgi?id=1320470Signed-off-by: NJiri Denemark <jdenemar@redhat.com>