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/monitor.c b/monitor.c
index bd63768..37b9d20 100644
--- a/monitor.c
+++ b/monitor.c
@@ -31,7 +31,7 @@
 #include "hw/watchdog.h"
 #include "hw/loader.h"
 #include "gdbstub.h"
-#include "net.h"
+#include "net/net.h"
 #include "net/slirp.h"
 #include "qemu-char.h"
 #include "ui/qemu-spice.h"