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/tap-linux.c b/net/tap-linux.c
index 3eaedc4..3de7b3b 100644
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -23,8 +23,9 @@
  * THE SOFTWARE.
  */
 
+#include "tap_int.h"
+#include "tap-linux.h"
 #include "net/tap.h"
-#include "net/tap-linux.h"
 
 #include <net/if.h>
 #include <sys/ioctl.h>