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/hw/virtio-net.c b/hw/virtio-net.c
index 108ce07..dc7c6d6 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -13,7 +13,7 @@
 
 #include "iov.h"
 #include "virtio.h"
-#include "net.h"
+#include "net/net.h"
 #include "net/checksum.h"
 #include "net/tap.h"
 #include "qemu-error.h"