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/ppc_prep.c b/hw/ppc_prep.c
index f6ffb59..25cc049 100644
--- a/hw/ppc_prep.c
+++ b/hw/ppc_prep.c
@@ -26,7 +26,7 @@
 #include "pc.h"
 #include "serial.h"
 #include "fdc.h"
-#include "net.h"
+#include "net/net.h"
 #include "sysemu.h"
 #include "isa.h"
 #include "pci/pci.h"