xen-pt: fix Negative array index read

Coverity spot:
Function xen_pt_bar_offset_to_index() may return a negative
value (-1) which is used as an index to d->io_regions[] down
the line.

Let's pass index directly as an argument to
xen_pt_bar_reg_parse().

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>
