qemu: Move some functions to qemu_capspriv.h

This header file has been created so that we can expose
internal functions to the test suite without making them
public: those in qemu_capabilities.h bearing the comment

  /* Only for use by test suite */

are obvious candidates for being moved over.

tests/qemuhelptest.c

@@ -8,6 +8,10 @@
 # include <stdlib.h>
 
 # include "qemu/qemu_capabilities.h"
+# define __QEMU_CAPSPRIV_H_ALLOW__
+# include "qemu/qemu_capspriv.h"
+# undef __QEMU_CAPSPRIV_H_ALLOW__
+
 # include "viralloc.h"
 # include "virstring.h"