xenserver: Remove cacert when user reconfigures the controller

xenserver: Remove cacert when user reconfigures the controller

If a user moves from one controller to another, we did not remove the
cacert.  This prevents the switch from connecting to the new controller.
To ease confusion, we now delete the cacert when the user changes or
removes the controller in xsconsole.

Note: This commit has a minor security issue, since we do not remove
trust for the old certificate until the switch is restarted.  In
general, users should only be connected to trusted servers, so the
impact should be low.  Fixes this would require larger changes to the
vconn-ssl code, which we don't want to do so late in the release cycle.

Bug #1457