xen/pt: xen_host_pci_config_read returns -errno, not -1 on failure

However the init routines assume that on errors the return
code is -1 (as the libxc API is) - while those xen_host_* routines follow
another paradigm - negative errno on return, 0 on success.

Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
