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: James Harper <james.harper@ejbdigital.com.au>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>