The XenAPI driver was removed during the Victoria release [1], while the
libvirt+xen driver has been removed in the Wallaby release [2]. Remove
references to Xen from DevStack since its all a no-op now.
[1] I42b302afbb1cfede7a0f7b16485a596cd70baf17
[2] I73305e82da5d8da548961b801a8e75fb0e8c4cf1
Change-Id: If7055feb88391f496a5e5e4c72008bf0050c5356
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>