console: allow log level threshold adjustments

console: allow log level threshold adjustments

... from serial console so that one doesn't always need to reboot to
see more / less messages.

Note that upper thresholds are sticky, i.e. while they get adjusted
upwards when the lower threshold would otherwise end up above the upper
one, they don't get adjusted when reducing the lower one. Full
flexibility is available only via a future sysctl interface.

Note further that (meaningless) large threshold values aren't being
rejected, for the sake of not adding more checks to the code than are
really necessary for safe operation.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>