network driver: log error and abort network startup when radvd isn't found

This is detailed in:

  https://bugzilla.redhat.com/show_bug.cgi?id=688957

Since radvd is executed by daemonizing it, the attempt to exec the
radvd binary doesn't happen until after libvirtd has already received
an exit code from the intermediate forked process, so no error is
detected or logged by __virExec().

We can't require radvd as a prerequisite for the libvirt package (many
installations don't use IPv6, so they don't need it), so instead we
add in a check to verify there is an executable radvd binary prior to
trying to exec it.

src/network/bridge_driver.c

@@ -689,6 +689,14 @@ networkStartRadvd(virNetworkObjPtr network)
 
     network->radvdPid = -1;
 
+    if (access(RADVD, X_OK) < 0) {
+        virReportSystemError(errno,
+                             _("Cannot find %s - "
+                               "Possibly the package isn't installed"),
+                             RADVD);
+        goto cleanup;
+    }
+
     if ((err = virFileMakePath(NETWORK_PID_DIR)) != 0) {
         virReportSystemError(err,
                              _("cannot create directory %s"),