partially fix #3056: try to cancel backup without uuid

partially fix #3056: try to cancel backup without uuid

if the 'backup' qmp call itself times out or fails, we still want to
try to cancel the backup, else it can happen that there is still
a backup running even when vzdump thinks it was canceled

qapi docs says that backup cancel always returns success, even
if no backup is running

since we hold a global and a per vm lock for the backup, this should be
ok, since we should not reach this code without that lock

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>