Revert "Error out on missing machine type in machine configs"

Revert commit 55e6d8cd.

This fix for https://bugzilla.redhat.com/show_bug.cgi?id=1267256
unconditionally required a machine type for all machine types
even though qemu is the only emulator using them.

Revert it to fix persistent configs for drivers with no machine type:
https://www.redhat.com/archives/libvir-list/2016-February/msg01228.html

src/conf/domain_conf.c

@@ -14851,12 +14851,6 @@ virDomainDefParseXML(xmlDocPtr xml,
             goto error;
         }
         VIR_FREE(capsdata);
-    } else {
-        if (!def->os.machine) {
-            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                           _("Missing machine type"));
-            goto error;
-        }
     }
 
     /* Extract domain name */