log: Introduce a simple printf-like logging engine

It's supposed to be used by parasite and restorer blob. It
has API equal to the core one -- with setfd, set_loglevel and
(the main thing) print_on_level fn. It currently supports only
strings, decimal and hex numbers (int and long).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>