Remove the xend driver

xend was deprecated in Xen 4.2 and removed from the Xen sources
before the Xen 4.5 release. The last Xen release to contain xend
was Xen 4.4, which was retired upstream in March 2017.

Remove xend support from libvirt since it is unrealistic to use
modern libvirt with ancient Xen.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>