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/net/queue.c b/net/queue.c
index 254f280..542c549 100644
--- a/net/queue.c
+++ b/net/queue.c
@@ -23,7 +23,7 @@
 
 #include "net/queue.h"
 #include "qemu-queue.h"
-#include "net.h"
+#include "net/net.h"
 
 /* The delivery handler may only return zero if it will call
  * qemu_net_queue_flush() when it determines that it is once again able