xenserver: Fix ovs-vsctl in built RPM by defining /etc as sysconfdir.

xenserver: Fix ovs-vsctl in built RPM by defining /etc as sysconfdir.

By default, the "configure" script picks a sysconfdir of $prefix/etc,
which works out to /usr/etc in our case.  That's wrong, of course--it
should be /etc--but we didn't notice until now because sysconfdir was
only used in ovs-vsctl, which in turn wasn't used at all on a XenServer
system until recently.

This bug is present on all branches, but it is only potentially visible
on "master" and "xs5.7", since only those have ovs-vsctl.  It is only
actually visible on "xs5.7", since that is the only branch where the
system uses ovs-vsctl itself (from /etc/xensource/scripts/vif), but this
is being committed to master in case we start using ovs-vsctl there too.

Reported-by: Henrik Amren <henrik@nicira.com>