Increase max floppy drives to 4

I suppose this patch will be highly controversial... Feel free to commit it or not.
It is required to successfully pass POST in the firmware, but the firmware will work without this patch.

Index: hw/fdc.h
===================================================================
RCS file: /sources/qemu/qemu/hw/fdc.h,v
retrieving revision 1.1

diff -u -r1.1 fdc.h
@@ -1,5 +1,5 @@
 /* fdc.c */
-#define MAX_FD 2
+#define MAX_FD 4
 extern BlockDriverState *fd_table[MAX_FD];
 
 typedef struct fdctrl_t fdctrl_t;