fix off-by-one error in pci_piix3_xen_ide_unplug

Fix off-by-one error when unplugging disks, which would otherwise leave the last ATA disk plugged, with obvious consequences. Also rewrite loop to be more readable.
Signed-off-by: NJames Harper <james.harper@ejbdigital.com.au>
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>

hw/ide/piix.c

@@ -171,11 +171,11 @@ int pci_piix3_xen_ide_unplug(DeviceState *dev)
 {
     PCIIDEState *pci_ide;
     DriveInfo *di;
-    int i = 0;
+    int i;
 
     pci_ide = PCI_IDE(dev);
 
-    for (; i < 3; i++) {
+    for (i = 0; i < 4; i++) {
         di = drive_get_by_index(IF_IDE, i);
         if (di != NULL && !di->media_cd) {
             BlockBackend *blk = blk_by_legacy_dinfo(di);