defaults: split default_drive

Split default_drive into default_{floppy,cdrom,sdcard}.
Also add QEMUMachine flags to disable them per machine.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/boards.h b/hw/boards.h
index 8fe0fbc..e1beda3 100644
--- a/hw/boards.h
+++ b/hw/boards.h
@@ -22,7 +22,10 @@
     int no_serial:1,
         no_parallel:1,
         use_virtcon:1,
-        no_vga:1;
+        no_vga:1,
+        no_floppy:1,
+        no_cdrom:1,
+        no_sdcard:1;
     int is_default;
     GlobalProperty *compat_props;
     struct QEMUMachine *next;