Don't copy old machines from a domain which has none

If the the qemu and kvm binaries are the same, we don't include machine
types in the kvm domain info.

However, the code which refreshes the machine types info from the
previous capabilities structure first looks at the kvm domain's info,
finds it matches and then copies the empty machine types list over
for the top-level qemu domain.

That doesn't make sense, we shouldn't copy an empty machin types list.

* src/qemu/qemu_conf.c: qemudGetOldMachinesFromInfo(): don't copy an
  empty machine types list.

src/qemu/qemu_conf.c

@@ -505,6 +505,9 @@ qemudGetOldMachinesFromInfo(virCapsGuestDomainInfoPtr info,
     virCapsGuestMachinePtr *list;
     int i;
 
+    if (!info->nmachines)
+        return 0;
+
     if (!info->emulator || !STREQ(emulator, info->emulator))
         return 0;