Introduce vircommandpriv.h for functions used by tests

So far it's just virCommandSetDryRun.

tests/virkmodtest.c

@@ -23,7 +23,8 @@
 #ifdef __linux__
 
 # include <stdlib.h>
-# include "vircommand.h"
+# define __VIR_COMMAND_PRIV_H_ALLOW__
+# include "vircommandpriv.h"
 # include "virkmod.h"
 # include "virstring.h"
 

tests/virnetdevbandwidthtest.c

@@ -21,7 +21,8 @@
 #include <config.h>
 
 #include "testutils.h"
-#include "vircommand.h"
+#define __VIR_COMMAND_PRIV_H_ALLOW__
+#include "vircommandpriv.h"
 #include "virnetdevbandwidth.h"
 #include "netdev_bandwidth_conf.c"