cfm: Log fault status changes more informatively.

cfm: Log fault status changes more informatively.

Until now, fault status changes just log the new status.  This means that
the administrator has to find two consecutive status change messages to
see what actually changed.

This commit changes the log message format to prefix new faults with '+'
and faults that disappeared with '-'.  Existing faults that are still
present are not prefixed.

This also simplifies the code a little by making ds_put_cfm_fault()
put spaces before fault names instead of after.

Signed-off-by: Ben Pfaff <blp@nicira.com>