util: set max wait for IPv6 DAD to 20 seconds

This was originally set to 5 seconds, but times of 5.5 to 7 seconds
were experienced. Since it's an arbitrary number intended to prevent
an infinite hang, having it a bit too high won't hurt anything, and 20
seconds looks to be adequate (i.e. I think/hope we don't need to make
it tunable in libvirtd.conf)

src/util/virnetdev.c

@@ -96,7 +96,7 @@ VIR_LOG_INIT("util.netdev");
 # define FEATURE_BIT_IS_SET(blocks, index, field)        \
     (FEATURE_WORD(blocks, index, field) & FEATURE_FIELD_FLAG(index))
 #endif
-#define VIR_DAD_WAIT_TIMEOUT 5 /* seconds */
+#define VIR_DAD_WAIT_TIMEOUT 20 /* seconds */
 
 typedef enum {
     VIR_MCAST_TYPE_INDEX_TOKEN,