feat(instances): a VM's destruction says what goes, and keeps what a kept file needs

The Destroy tab was a sub-tab with one sentence and a link; its
confirmation page said only "This action is irreversible!" over a
"Remove Instance's data" switch. The tab now shows the state and
"Review destruction". The owner chose the confirmation layout from
three rendered options: the VM and its host, that its definition,
saved state and snapshot records go and that a running VM is forced
off, two plans for the files (delete the disk files, or keep them), the
VM's disks, what is always kept, the NVRAM choice, and Cancel.

Codex reviewed destroying; this is fixed:

- A disk file that cannot be deleted keeps the files below it, which
  it needs: they were deleted after it, leaving it broken. The files
  are deleted overlays first (split_disk_paths_by_use orders them by
  how many files each grows from), and if the files a kept one needs
  cannot be read, none of the rest goes. A message names each file
  kept so, and a failed deletion says the VM is removed.

vrtManager/instance.py

@@ -1705,10 +1705,18 @@ class wvmInstance(wvmConnect):
         used_elsewhere = self.paths_used_by_other_domains(own)
         # a kept file keeps the files below it
         for path in [path for path in own if path in used_elsewhere]:
-            while (path := self._backing_path(path)) and path not in used_elsewhere:
+            while (path := self.backing_path(path)) and path not in used_elsewhere:
                 used_elsewhere.add(path)
+        def below(path):
+            """How many files the path grows from: an overlay is deleted before
+            the files it needs, so one that cannot be deleted keeps them."""
+            seen = set()
+            while (path := self.backing_path(path)) and path not in seen:
+                seen.add(path)
+            return len(seen)
+
         return (
-            [path for path in own if path not in used_elsewhere],
+            sorted((path for path in own if path not in used_elsewhere), key=below, reverse=True),
             [path for path in own if path in used_elsewhere],
         )
 
@@ -1739,7 +1747,7 @@ class wvmInstance(wvmConnect):
                 points.update(self._disk_sources(saved).values())
         files = list(tops)
         for path in tops:
-            while (path := self._backing_path(path)) in points and path not in files:
+            while (path := self.backing_path(path)) in points and path not in files:
                 files.append(path)
         grew = True
         while grew:
@@ -1748,14 +1756,14 @@ class wvmInstance(wvmConnect):
                 if top in files:  # reached by a branch in this pass
                     continue
                 branch, path = [top], top
-                while (path := self._backing_path(path)) in points | overlays and path not in files:
+                while (path := self.backing_path(path)) in points | overlays and path not in files:
                     branch.append(path)
                 if path in files:
                     files += branch
                     grew = True
         return files[len(tops):] + memory
 
-    def _backing_path(self, path):
+    def backing_path(self, path):
         """The backing file of a volume; None if it has none or is in no pool.
         Any other error is raised: a chain cut short would let a file a kept
         one needs count as deletable."""
@@ -2026,7 +2034,7 @@ class wvmInstance(wvmConnect):
             # do not keep each other, but a file in use keeps the files below it
             in_use = self.paths_in_use(own=left)
             for path in [path for path in left if path in in_use]:
-                while (path := self._backing_path(path)) and path not in in_use:
+                while (path := self.backing_path(path)) and path not in in_use:
                     in_use.add(path)
             left = [path for path in left if path not in in_use]
         return left