Increase the NBD buffer size.

When qemu-img talks to qemu-nbd, the buffer size used by the server is too
small to support many operations, including 'convert'. This is a shame. This
patch bumps the buffer size to a reasonable 4MB, allowing you to qemu-img
convert from a qemu-nbd source.

Signed-off-by: Adin Scannell <adin@scannell.ca>

Index: qemu/nbd.h
===================================================================
--- qemu.orig/nbd.h	2013-01-17 10:57:01.536404694 -0600
+++ qemu/nbd.h	2013-01-17 10:57:01.528404694 -0600
@@ -57,7 +57,7 @@
 
 #define NBD_DEFAULT_PORT	10809
 
-#define NBD_BUFFER_SIZE (1024*1024)
+#define NBD_BUFFER_SIZE (4*1024*1024)
 
 ssize_t nbd_wr_sync(int fd, void *buffer, size_t size, bool do_read);
 int tcp_socket_outgoing(const char *address, uint16_t port);
