I add this method following the style of libvirt_vm.attachXXX and libvirt_vm.detachXXX.

Yes, it does the same work with virsh.attach_disk.
But libvirt_vm.attach_disk is a method for libvirt_vm class and virsh.attach_disk is a function for virsh module.

I think this functionality for libvirt_vm is necessary.

:)