xenserver: Renice netback process to priority 0 by default.

Under heavy VM network load, we have observed that ovs-vswitchd can be
starved for CPU time, which prevents flows from being set up.  This can
in turn cause connections to XAPI in Dom0 to time out (among other issues).

It is probably not necessary to renice netback all the way to priority 0
as done in this commit.  That is simply the value that we have tested.  QA
has not reported any ill side-effects of this choice of value (yet).  One
reasonable alternative, should any problems be noticed, would be to leave
netback at its default -5 priority and simply boost ovs-vswitchd's priority
to say -6 or -7.

Bug #1656.