virt: add comment that we need to use sscanf()

Follow-up for 13e0f9fe83.
See PR #7890 and comment in PR #7581.

src/basic/virt.c

@@ -222,6 +222,8 @@ static int detect_vm_xen_dom0(void) {
         if (r == 0) {
                 unsigned long features;
 
+                /* Here, we need to use sscanf() instead of safe_atoul()
+                 * as the string lacks the leading "0x". */
                 r = sscanf(domcap, "%lx", &features);
                 if (r == 1) {
                         r = !!(features & (1U << XENFEAT_dom0));