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/mipsnet.c b/hw/mipsnet.c
index bece332..bb752d3 100644
--- a/hw/mipsnet.c
+++ b/hw/mipsnet.c
@@ -1,5 +1,5 @@
 #include "hw.h"
-#include "net.h"
+#include "net/net.h"
 #include "trace.h"
 #include "sysbus.h"