hmp: delvm: use hmp_handle_error

This gives us the consistent 'Error:' prefix added in 66363e9a,
which helps users like libvirt who still need to scrape hmp error
messages to detect failure.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>
Message-Id: <984634226fa14316641f31f84e7dd0bb770bd94c.1556141033.git.crobinso@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

hmp.c

@@ -1480,10 +1480,11 @@ void hmp_delvm(Monitor *mon, const QDict *qdict)
     const char *name = qdict_get_str(qdict, "name");
 
     if (bdrv_all_delete_snapshot(name, &bs, &err) < 0) {
-        error_reportf_err(err,
-                          "Error while deleting snapshot on device '%s': ",
-                          bdrv_get_device_name(bs));
+        error_prepend(&err,
+                      "deleting snapshot on device '%s': ",
+                      bdrv_get_device_name(bs));
     }
+    hmp_handle_error(mon, &err);
 }
 
 void hmp_info_snapshots(Monitor *mon, const QDict *qdict)