snapshot_destroy: make interface agnostic to source/dest

snapshot_destroy: make interface agnostic to source/dest

Also drop the 'method' parameter which is not used consistently (e.g. ignored
in the later half of the very same function, in snapshot_exist, vm_exists,...),
and not documented. Simply rely on the presence of the IP address as is done in
many other places already.

In snapshot_add, there is no need to try and destroy the snapshot on the
destination (did happen previously, because $dest was passed along), because we
just failed to create the snapshot on the source side.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>