domain_conf: Resolve Coverity CHECKED_RETURN

Commit id '0d36a5d05' modified the code slightly, but removed the
return value check thus causing Coverity to complain that this call
was the only one where the return value wasn't checked. Since nothing
was done previously if there was a failure, just use ignore_value here
to pacify Coverity

src/conf/domain_conf.c

@@ -19237,7 +19237,7 @@ virDomainDefFormatInternal(virDomainDefPtr def,
         virDomainResourceDefFormat(buf, def->resource);
 
     if (def->sysinfo)
-        virSysinfoFormat(buf, def->sysinfo);
+        ignore_value(virSysinfoFormat(buf, def->sysinfo));
 
     if (def->os.bootloader) {
         virBufferEscapeString(buf, "<bootloader>%s</bootloader>\n",