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

hw/xen/xen_pt.c

@@ -729,7 +729,7 @@ static int xen_pt_initfn(PCIDevice *d)
 
     /* Initialize virtualized PCI configuration (Extended 256 Bytes) */
     if (xen_host_pci_get_block(&s->real_device, 0, d->config,
-                               PCI_CONFIG_SPACE_SIZE) == -1) {
+                               PCI_CONFIG_SPACE_SIZE) < 0) {
         xen_host_pci_device_put(&s->real_device);
         return -1;
     }