xenserver: Kill bond slaves' dhclients when bringing up bond master.

xenserver: Kill bond slaves' dhclients when bringing up bond 

This fixes the converse of the problem addressed by commit fe19e820
"xenserver: Kill bond master's dhclient when bringing up bond slave".  In
that commit's log message, I claimed that the converse was not a problem,
but I was wrong.  I must have screwed up in testing, because it really is
a problem.  This commit fixes it.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Dominic Curran <dominic.curran@citrix.com>
Reported-by: Michael Mao <mmao@nicira.com>
Bug #2668.