References: bsc#1167608
unbound limits for max_event_channels
1023 is too low for a three digit value of vcpus
it is difficult to make the value depend on the number of vcpus
adding devices at runtime also needs event channels

But, having an unbound value (of 128k) may have a negative effect on XSA-344.

Therefore, just let the built-in default depend on the number of vcpus.

Index: xen-4.17.0-testing/tools/libs/light/libxl_create.c
===================================================================
--- xen-4.17.0-testing.orig/tools/libs/light/libxl_create.c
+++ xen-4.17.0-testing/tools/libs/light/libxl_create.c
@@ -263,7 +263,7 @@ int libxl__domain_build_info_setdefault(
             b_info->iomem[i].gfn = b_info->iomem[i].start;
 
     if (!b_info->event_channels)
-        b_info->event_channels = 1023;
+        b_info->event_channels = max(1023, 8 * b_info->max_vcpus + 511);
 
     rc = libxl_get_physinfo(CTX, &info);
     if (rc) {