Make xend-config.sxp optional

Make xend-config.sxp optional

With Ganeti imposing no requirement on Xen versions, the configuration
file that should be present varies greatly on the choice of toolstack
and Xen version. xend-config.sxp was considered obligatory, but in
higher versions and with xl, it is superseded by xl.conf and considered
deprecated. This patch makes it have the same status as xl.conf - Ganeti
will distribute it but not require its presence.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>