tools/libxenstat: don't lose all VIFs when dom0 has no Linux bridge

xenstat_collect_networks() looks for a Linux bridge (a
/sys/class/net/*/bridge directory) so that, with bonding, dom0 can be
given the bridge's counters. When there is none, as on hosts using Open
vSwitch (the XCP-ng default), routed networking or only libvirt's
virbr0 (which is excluded on purpose), getBridge() leaves devBridge
empty. strstr(iface, "") then matches every interface, and devNoBridge
is just "p", so every VIF with a standard name (vifN.M, vifN.M-emu)
takes the bridge branch instead of being attached to its domain. Every
domain ends up with zero networks, and xentop shows NETS 0.

Only take the bridge branch when a bridge was actually found.

Fixes: 1fd93d4f8c ("xenstat: Linux dom0 statistics for case we use network bonding")
Assisted-by: Claude-Code:claude-opus-5-5
Signed-off-by: Olivier Lambert <olivier.lambert@vates.tech>
Tested-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

tools/libs/stat/xenstat_linux.c

@@ -322,7 +322,8 @@ int xenstat_collect_networks(xenstat_node * node)
 
 		/* If the device parsed is network bridge and both tx & rx packets are zero, we are most */
 		/* likely using bonding so we alter the configuration for dom0 to have bridge stats */
-		if ((strstr(iface, devBridge) != NULL) &&
+		if ((devBridge[0] != '\0') &&
+		    (strstr(iface, devBridge) != NULL) &&
 		    (strstr(iface, devNoBridge) == NULL) &&
 		    ((domain = xenstat_node_domain(node, 0)) != NULL)) {
 			for (i = 0; i < domain->num_networks; i++) {