blockdev: Fix regression in -drive if=scsi,index=N

Before commit 622b520f, index=12 meant bus=1,unit=5.

Since the commit, it means bus=0,unit=12.  The drive is created, but
not the guest device.  That's because the controllers we use with
if=scsi drives (lsi53c895a and esp) support only 7 units, and
scsi_bus_legacy_handle_cmdline() ignores drives with unit numbers
exceeding that limit.

Changing the mapping of index to bus, unit is a regression.  Breaking
-drive invocations that used to work just makes it worse.

Revert the part of commit 622b520f that causes this, and clean up
some.

Note that the fix only affects if=scsi.  You can still put more than 7
units on a SCSI bus with -device & friends.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/ide.h

@@ -4,6 +4,8 @@
 #include "isa.h"
 #include "pci.h"
 
+#define MAX_IDE_DEVS	2
+
 /* ide-isa.c */
 ISADevice *isa_ide_init(int iobase, int iobase2, int isairq,
                         DriveInfo *hd0, DriveInfo *hd1);