migration: fix id leak regression

This leak was introduced in commit
581f08ba.

(it stands out quickly with ASAN once the rest of the leaks are also
removed from make check with this series)

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170221141451.28305-31-marcandre.lureau@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

migration/savevm.c

@@ -688,6 +688,7 @@ int vmstate_register_with_alias_id(DeviceState *dev, int instance_id,
 
                 return -1;
             }
+            g_free(id);
 
             se->compat = g_new0(CompatEntry, 1);
             pstrcpy(se->compat->idstr, sizeof(se->compat->idstr), vmsd->name);