These XMLs live in a separate directory, there's no need for them
to have a special prefix in addition. It also doesn't play nicely
with ':e' completion in Vim, finding proper file based on
qemuxml2argvtest.c is also needlessly complicated.

The files were renamed using the following commands. From
qemuxml2argvdata:

  for i in qemuxml2argv-*.xml; do mv $i ${i#qemuxml2argv-}; done

and then (to fix broken symlinks) from qemuxml2argvdata and
qemuxml2xmloutdata:

  for i in $(find . -xtype l); do \
      ln -sf $(readlink $i | sed 's/qemuxml2argv-//') $i;
  done
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

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