coverage: Make ovs-appctl command more useful and less alarming.

coverage: Make ovs-appctl command more useful and less alarming.

I've had a few complaints that ovs-vswitchd logs its coverage counters
at WARN level, but this is mainly wrong: ovs-vswitchd only logs coverage
counters at WARN level when the "coverage/log" command is used through
ovs-appctl.  This was even documented.

The reason to log at such a high level was to make it fairly certain that
these messages specifically requested by the admin would not be filtered
out before making it to the log.  But it's even better if the admin just
gets the coverage counters as a reply to the ovs-appctl command.  So that
is what this commit does.

This commit also improves the documentation of the ovs-appctl command.

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