[nexus] Consider SagaUnwound instances Failed

Currently, instances whose active VMM is `SagaUnwound` appear externally
as `Stopped`. We decided to report them as `Stopped` because start sagas
are permitted to run for instances with `SagaUnwound` active VMMs, and
--- at the time when the `SagaUnwound` VMM state was introduced,
`Failed` instances could not be started. However, #6455 added the
ability to restart `Failed` instances, and #6652 will permit them to be
stopped. Therefore, we should recast instances with `SagaUnwound` active
VMMs as `Failed`: they weren't asked politely to stop; instead, we
attempted to start them and something went wrong...which sounds like
`Failed` to me.

This becomes more important in light of #6638: if we will attempt
automatically restart such instances, they should definitely appear
to be `Failed`. The distinction between `Failed` and `Stopped` becomes
that `Failed` means "this thing isn't running, but it's supposed to be;
we may try to fix that for you if permitted to do so", while `Stopped`
means "this thing isn't running and that's fine, because you asked for
it to no longer be running". Thus, this commit changes `SagaUnwound`
VMMs to appear `Failed` externally.

nexus/db-queries/src/db/datastore/instance.rs

@@ -154,20 +154,20 @@ impl InstanceAndActiveVmm {
                 InstanceState::Vmm,
                 Some(VmmState::Stopped | VmmState::Destroyed),
             ) => external::InstanceState::Stopping,
-            // - An instance with a "saga unwound" VMM, on the other hand, can
-            //   be treated as "stopped", since --- unlike "destroyed" --- a new
-            //   start saga can run at any time by just clearing out the old VMM
-            //   ID.
-            (InstanceState::Vmm, Some(VmmState::SagaUnwound)) => {
-                external::InstanceState::Stopped
-            }
             // - An instance with a "failed" VMM should *not* be counted as
             //   failed until the VMM is unlinked, because a start saga must be
-            //   able to run "failed" instance. Until then, it will continue to
-            //   appear "stopping".
+            //   able to run for a "failed" instance. Until then, it will
+            //   continue to appear "stopping".
             (InstanceState::Vmm, Some(VmmState::Failed)) => {
                 external::InstanceState::Stopping
             }
+            // - An instance with a "saga unwound" VMM, on the other hand, can
+            //   be treated as "failed", since --- unlike an instance with a
+            //   "failed" active VMM --- a new start saga can run at any time by
+            //   just clearing out the old VMM ID.
+            (InstanceState::Vmm, Some(VmmState::SagaUnwound)) => {
+                external::InstanceState::Failed
+            }
             // - An instance with no VMM is always "stopped" (as long as it's
             //   not "starting" etc.)
             (InstanceState::NoVmm, _vmm_state) => {