xen: encourage use of xen:///system URI as preferred format

xen: encourage use of xen:///system URI as preferred format

Historically we have used a bare xen:/// URI for connecting to the
legacy Xen driver. The new libxl Xen driver follows the new practice
of allowing '/system' as a path, as well as bare '/' for compat with
the old Xen driver.

This documents xen:///system as the preferred format for Xen, leaving
xen:/// as an undocumented feature just for back-compat.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>