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>

blockdev.c

@@ -32,8 +32,22 @@ static const char *const if_name[IF_COUNT] = {
 };
 
 static const int if_max_devs[IF_COUNT] = {
-    [IF_IDE] = MAX_IDE_DEVS,
-    [IF_SCSI] = MAX_SCSI_DEVS,
+    /*
+     * Do not change these numbers!  They govern how drive option
+     * index maps to unit and bus.  That mapping is ABI.
+     *
+     * All controllers used to imlement if=T drives need to support
+     * if_max_devs[T] units, for any T with if_max_devs[T] != 0.
+     * Otherwise, some index values map to "impossible" bus, unit
+     * values.
+     *
+     * For instance, if you change [IF_SCSI] to 255, -drive
+     * if=scsi,index=12 no longer means bus=1,unit=5, but
+     * bus=0,unit=12.  With an lsi53c895a controller (7 units max),
+     * the drive can't be set up.  Regression.
+     */
+    [IF_IDE] = 2,
+    [IF_SCSI] = 7,
 };
 
 /*