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-win32.c b/net/tap-win32.c
index f9bd741..e37d8ee 100644
--- a/net/tap-win32.c
+++ b/net/tap-win32.c
@@ -26,11 +26,11 @@
  *  distribution); if not, see <http://www.gnu.org/licenses/>.
  */
 
-#include "tap.h"
+#include "tap_int.h"
 
 #include "qemu-common.h"
 #include "clients.h"            /* net_init_tap */
-#include "net.h"
+#include "net/net.h"
 #include "sysemu.h"
 #include "qemu-error.h"
 #include <stdio.h>