migrate: add remote migration handling

migrate: add remote migration handling

remote migration uses a websocket connection to a task worker running on
the target node instead of commands via SSH to control the migration.
this websocket tunnel is started earlier than the SSH tunnel, and allows
adding UNIX-socket forwarding over additional websocket connections
on-demand.

the main differences to regular intra-cluster migration are:
- source VM config and disks are only removed upon request via --delete
- shared storages are treated like local storages, since we can't
assume they are shared across clusters (with potentical to extend this
by marking storages as shared)
- NBD migrated disks are explicitly pre-allocated on the target node via
tunnel command before starting the target VM instance
- in addition to storages, network bridges and the VMID itself is
transformed via a user defined mapping
- all commands and migration data streams are sent via a WS tunnel proxy
- pending changes and snapshots are discarded on the target side (for
  the time being)

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>