Require destination name when copying an instance on the same server

[MusicDin]

Removes unused code that determines generated name on copy or move (using copy).
It requires destination name to be provided, with only exception being when moving an instance to another remote (for example: lxc move c1 remote2:). In such case, the destination name is set to the source name.

Fixes #12338

[ru-fu]

This looks like it needs some doc updates to
https://documentation.ubuntu.com/lxd/en/latest/howto/move_instances/#
and
https://documentation.ubuntu.com/lxd/en/latest/howto/cluster_manage_instance/#move-an-instance

[tomponline]

This comment say what but doesn't explain the why.

[MusicDin]

Thanks for this one.

Snapshot suffix is removed to ensure consistency when copying/moving an instance (or snapshot) to a different remote (lxc move c1 remote2: or lxc move c1/snap0 remote2:).

However, I missed the fact, that this will trim the suffix even if the user provides the snapshot name in the destination name.

For example, the following commands produced the same error, although they should not:

lxc copy c1/snap0 c1/snap1
lxc copy c1/snap0 c1

# Error: Failed creating instance record: Add instance info to the database: This "instances" entry already exists`

Now, the snapshot name is trimmed only when destName is set from the source.

---

A bit unrelated, but is there a reason we do not output the instance name in the above error? (something like Database entry for instance "c1" already exists)

[MusicDin]

@ru-fu I've checked the docs and to me everything seems up to date. It is most likely that the PR title is misleading.

This PR requires the destination name to be provided if remote is not specified, as the command would fail anyway because the instance with that name already exists.

lxc copy c1

And it fixes the situation where an instance is created from a snapshot on a different remote:

lxc copy c1/snap0 remote2:

The above command would previously fail with Error: The character "/" is reserved for snapshots, so now we trim the snapshot name when destination name is not provided.

---

We had some code related to determining the generated instance name, which is no longer being used. To me it seems that lxc copy used to generate an instance name if destination name is not provided (similar to lxc init when instance name is not provided). I was not able to find anything related to that in the docs, but just to emphasize it here if any leftovers are found.

[ru-fu]

OK, great!

So lxc move c1 --target server1 is still going to work?

[MusicDin]

I've double checked and it is working

[tomponline]

Do we need to consider copying a snapshot with --target specified too?