domain_conf: Resolve Coverity COPY_PASTE_ERROR

Seems when commit id 'ea130e3b' added the checks to ensure each of
the hard_limit, soft_limit, and swap_hard_limit wasn't set at
VIR_DOMAIN_MEMORY_PARAM_UNLIMITED - a copy/paste error of using
the 'hard_limit' for each comparison was done. Adjust the code.

src/conf/domain_conf.c

@@ -18088,9 +18088,9 @@ virDomainDefFormatInternal(virDomainDefPtr def,
     if ((def->mem.hard_limit &&
          def->mem.hard_limit != VIR_DOMAIN_MEMORY_PARAM_UNLIMITED) ||
         (def->mem.soft_limit &&
-         def->mem.hard_limit != VIR_DOMAIN_MEMORY_PARAM_UNLIMITED) ||
+         def->mem.soft_limit != VIR_DOMAIN_MEMORY_PARAM_UNLIMITED) ||
         (def->mem.swap_hard_limit &&
-         def->mem.hard_limit != VIR_DOMAIN_MEMORY_PARAM_UNLIMITED) ||
+         def->mem.swap_hard_limit != VIR_DOMAIN_MEMORY_PARAM_UNLIMITED) ||
         def->mem.min_guarantee) {
         virBufferAddLit(buf, "<memtune>\n");
         virBufferAdjustIndent(buf, 2);