PCI: bus scan adjustments

As done elsewhere, the ns16550 code shouldn't look at non-zero
functions of a device if that isn't multi-function.

Also both there and in pass-through's _scan_pci_devices() skip looking
at non-zero functions when the device at function zero doesn't exist.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -469,21 +469,28 @@ static int
 pci_uart_config (struct ns16550 *uart, int skip_amt, int bar_idx)
 {
     uint32_t bar, len;
-    int b, d, f;
+    int b, d, f, nextf;
 
     /* NB. Start at bus 1 to avoid AMT: a plug-in card cannot be on bus 0. */
     for ( b = skip_amt ? 1 : 0; b < 0x100; b++ )
     {
         for ( d = 0; d < 0x20; d++ )
         {
-            for ( f = 0; f < 0x8; f++ )
+            for ( f = 0; f < 8; f = nextf )
             {
+                nextf = (f || (pci_conf_read16(0, b, d, f, PCI_HEADER_TYPE) &
+                               0x80)) ? f + 1 : 8;
+
                 switch ( pci_conf_read16(0, b, d, f, PCI_CLASS_DEVICE) )
                 {
                 case 0x0700: /* single port serial */
                 case 0x0702: /* multi port serial */
                 case 0x0780: /* other (e.g serial+parallel) */
                     break;
+                case 0xffff:
+                    if ( !f )
+                        nextf = 8;
+                    /* fall through */
                 default:
                     continue;
                 }
--- a/xen/drivers/passthrough/pci.c
+++ b/xen/drivers/passthrough/pci.c
@@ -628,7 +628,11 @@ static int __init _scan_pci_devices(stru
             for ( func = 0; func < 8; func++ )
             {
                 if ( pci_device_detect(pseg->nr, bus, dev, func) == 0 )
+                {
+                    if ( !func )
+                        break;
                     continue;
+                }
 
                 pdev = alloc_pdev(pseg, bus, PCI_DEVFN(dev, func));
                 if ( !pdev )