pc, pc64: disable all pci devices in pcicfginit()

Make sure all pci busmaster activity is disabled,
including MSI/MSI-X interrupts. Drivers will later
reenable once taking control of a device.

sys/src/9/pc/pcipc.c

@@ -716,6 +716,8 @@ pcicfginit(void)
 	if(pciroot == nil)
 		goto out;
 
+	pcireset();
+
 	if(nobios) {
 		uvlong mema;
 		ulong ioa;