Replace all instances of wwlog.Printf

wwlog provides named loggers for each level, which requires
less code and is clearer than wwlog.Printf. The code has
included a mix of both, but this commit consolidates existing
code on the per-level functions.

Signed-off-by: Jonathon Anderson <janderson@ciq.co>

internal/app/wwctl/kernel/list/main.go

@@ -14,7 +14,7 @@ func CobraRunE(cmd *cobra.Command, args []string) error {
 
 	kernels, err := kernel.ListKernels()
 	if err != nil {
-		wwlog.Printf(wwlog.ERROR, "%s\n", err)
+		wwlog.Error("%s\n", err)
 		os.Exit(1)
 	}