fix the 16-bit code segment's granularity

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/cstart.S b/cstart.S
index 938139c..b3bfc5d 100644
--- a/cstart.S
+++ b/cstart.S
@@ -29,7 +29,7 @@
         .quad 0
         .quad 0x00cf9b000000ffff // flat 32-bit code segment
         .quad 0x00cf93000000ffff // flat 32-bit data segment
-        .quad 0x008f9b0f0000ffff // 16-bit code segment at 0xF0000
+        .quad 0x000f9b0f0000ffff // 64K 16-bit code segment at 0xF0000
 gdt32_end:
 
 gdt32_descr: