logging: Lower logging level for coverage messages due to slow poll loops

logging: Lower logging level for coverage messages due to slow poll loops

By default, many OVS processes keep track of their time through a poll
loop.  If it takes an unusually long time (measured as some distance
from the mean), the processes will log stats it has been keeping about
coverage.  It was doing this at level WARN.

On Xen systems, syslog messages written at level INFO and higher are
written to /var/log/messages synchronously.  This would mean that there
would be dire messages that it took a few dozen milliseconds to go
through the loop, meanwhile, it would take up to 6(!) seconds writing
those.  Meanwhile, the process would do no other processing, which could
be quite serious in the case of a process such as ovs-vswitchd.
This problem was somewhat masked because the time used by this logging
was not used in the calculations for determining how long it was taking
to get through the loop.

This commit lowers the default log level for those coverage messages to
INFO.  On Xen systems, it raises the default level at which messages are
written to syslog to WARN.

Diagnosed and fixed with the help of Ian Campbell.