Move logging functions to a new file, log.c

Logging to file is going to add some further complexity that we don't
want to squeeze into util.c.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

isolation.c

@@ -42,6 +42,7 @@
 #include "util.h"
 #include "seccomp.h"
 #include "passt.h"
+#include "log.h"
 #include "isolation.h"
 
 /**