Change I4f97c05e2dec610af22a5150dd27696e1d767896 worked around
a change introduced in libvirt 1.3.3 where the script path on
a LibvirtConfigGuestInterface could not be the emptry string
because libvirt would literally take that as the path and couldn't
resolve it, when in fact it used to indicate to libvirt that the
script path is a noop. This has been fixed in libvirt 3.1.
On Ubuntu with libvirt<1.3.3, if the script path is None then
it defaults to /etc/qemu-ifup which is blocked by AppArmor.
So this change adds a conditional check when setting the script
path value based on the libvirt version so we can straddle releases.
Change-Id: I192c61b93bd3736fdfe16b6a6906d58997d3eef9
Closes-Bug: #1665698
Related-Bug: #1649527
a41d265a19