Return CDEMU parameters to ebda segment (instead of using globals).

The cdemu is setup in the boot stage after globals are made
    read-only.  So, globals can't be used.
Also, move hdcount from a global to BDA (it should be in BDA).
Don't attempt to set blksize/sector count in atapi_is_ready() - it's
    not useful.
diff --git a/src/boot.c b/src/boot.c
index 7b64e86..df3647d 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -152,8 +152,9 @@
             return;
         }
 
-        bootdrv = GET_GLOBAL(CDEMU.emulated_drive);
-        bootseg = GET_GLOBAL(CDEMU.load_segment);
+        u16 ebda_seg = get_ebda_seg();
+        bootdrv = GET_EBDA2(ebda_seg, cdemu.emulated_drive);
+        bootseg = GET_EBDA2(ebda_seg, cdemu.load_segment);
         /* Canonicalize bootseg:bootip */
         bootip = (bootseg & 0x0fff) << 4;
         bootseg &= 0xf000;