scsi: fix drive hotplug.

This patch fills the DriveInfo->unit after hotplugging a scsi disk.
It makes a difference when auto-assigning a scsi id, where unit was
left filled with '-1' instead of the actual scsi id.

With this patch applied the the drive naming logic in drive_init() works
as good as it did in previous releases.  Which means it works fine with
a single scsi bus.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/pci-hotplug.c

@@ -93,6 +93,7 @@ static int scsi_hot_add(DeviceState *adapter, DriveInfo *dinfo, int printinfo)
      */
     dinfo->unit = qemu_opt_get_number(dinfo->opts, "unit", -1);
     scsidev = scsi_bus_legacy_add_drive(scsibus, dinfo, dinfo->unit);
+    dinfo->unit = scsidev->id;
 
     if (printinfo)
         qemu_error("OK bus %d, unit %d\n", scsibus->busnr, scsidev->id);