syz-manager: check that coverage is enabled in ifaceprobe mode

Check coverage early.
Otherwise we do machine check w/o coverage, but then it's enabled
by ifaceprobe package for all programs anyway.

Author: dvyukov

syz-manager/manager.go

@@ -168,6 +168,9 @@ var (
 			if cfg.Sandbox != "none" {
 				return fmt.Errorf("sandbox \"%v\" is not supported (only \"none\")", cfg.Sandbox)
 			}
+			if !cfg.Coverage {
+				return fmt.Errorf("coverage is required")
+			}
 			return nil
 		},
 	}