Less anxiety-inducing return values

[hawkw]

Currently, the Vm::get() and Vm::state_watcher() methods return a
Result<{something}, VmError>. This means that they can return any
VmError variant, but the current implementation of these methods will
only ever return VmError::NotCreated.

This gave me a bit of anxiety when I noticed that we were presently
handling the VmError::WaitingForInit variant identically to
NotCreated, which seemed wrong as it implied that we could,
potentially, return an error code indicating that no instance has ever
been ensured when in fact, one has been ensured but is still being
initialized. I freaked out about this a bit here:
oxidecomputer/omicron#6726 (comment)

It turns out the current behavior is actually fine, since if the VM is
still initializing, we still allow accessing it and just return a
Starting state, which is correct. But the type signatures of these
functions allow them to return any of these errors, and forces their
callers to handle those errors, and that's the part we were doing
somewhat incorrectly (although, again, in practice it doesn't matter).

Commit 81de422 changes those functions to return an Option rather than
a Result. Now, we return None if no VM has ever been created, making
that the only error case that callers have to handle. There's no
longer any risk of the HTTP API handlers accidentally returning a "VM
never ensured" error when the VM is ensured-but-initializing. Also, we
can remove the NotCreated variant, since it's now no longer returned
anywhere.

Also, I noticed that the PUT /instance/state API route would return a
NoInstance error when trying to change the state returned a
VmError::WaitingToInitialize, which seemed potentially bad: this would
result in a sled-agent that tries to send a state change request to a
still-initializing VM to believe it's Permanently Gone, and mark it as
Failed, tear down the zone, and so on. Which seems rude of it!

I don't think this is likely to be a problem in practice since, IIRC,
both sled-agent and Nexus will not try to send state change requests to
instances that they understand to be still initializing, but it seemed
good to not return the INSTANCE IS PERMANENTLY GONE error code here.
Therefore, commit ce6bb51 changes this to return a 503, so the
sled-agent will just know it needs to wait for a bit.