net: reorganize headers

Move public headers to include/net, and leave private headers in net/.
Put the virtio headers in include/net/tap.h, removing the multiple copies
that existed.  Leave include/net/tap.h as the interface for NICs, and
net/tap_int.h as the interface for OS-specific parts of the tap backend.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hmp.c b/hmp.c
index 873962f..9b31e9f 100644
--- a/hmp.c
+++ b/hmp.c
@@ -14,7 +14,7 @@
  */
 
 #include "hmp.h"
-#include "net.h"
+#include "net/net.h"
 #include "qemu-char.h"
 #include "qemu-option.h"
 #include "qemu-timer.h"