hw: move private headers to hw/ subdirectories.

Many headers are used only in a single directory.  These can be
kept in hw/.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/audio/gusemu_mixer.c b/hw/audio/gusemu_mixer.c
index 816c58a..6d8d9ce 100644
--- a/hw/audio/gusemu_mixer.c
+++ b/hw/audio/gusemu_mixer.c
@@ -22,8 +22,8 @@
  * THE SOFTWARE.
  */
 
-#include "hw/gusemu.h"
-#include "hw/gustate.h"
+#include "gusemu.h"
+#include "gustate.h"
 
 #define GUSregb(position)  (*            (gusptr+(position)))
 #define GUSregw(position)  (*(GUSword *) (gusptr+(position)))