xen-pt: fix Out-of-bounds read

The array length of s->real_device.io_regions[] is
"PCI_NUM_REGIONS - 1".

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
