My commit 747761a7 (v1.2.15 only) dropped this bit of logic when filling
in a default arch in the XML:

-    /* First try to find one matching host arch */
-    for (i = 0; i < caps->nguests; i++) {
-        if (caps->guests[i]->ostype == ostype) {
-            for (j = 0; j < caps->guests[i]->arch.ndomains; j++) {
-                if (caps->guests[i]->arch.domains[j]->type == domain &&
-                    caps->guests[i]->arch.id == caps->host.arch)
-                    return caps->guests[i]->arch.id;
-            }
-        }
-    }

That attempt to match host.arch is important, otherwise we end up
defaulting to i686 on x86_64 host for KVM, which is not intended.
Duplicate it in the centralized CapsLookup function.

Additionally add some testcases that would have caught this.

https://bugzilla.redhat.com/show_bug.cgi?id=1219191