fix(instances): a snapshot needs free space in the VM's storage pools

A disk-only snapshot was taken without any space check, though its
overlays grow as the guest writes and a later delete merges them down;
a full pool pauses the guest and can leave the snapshot undeletable.
Only a memory snapshot was checked, for the memory alone.

Before a snapshot, each storage pool with a disk of the VM must have
free a share of those disks' virtual size (10%, at least 1 GiB), and
the pool of the memory file the VM's memory on top. Both values are on
the Settings page (Snapshot Free Percent, Snapshot Free Min GiB; 0
turns a rule off, an invalid value keeps the default). The pools are
refreshed first, and a failed refresh or an unreadable volume refuses
the snapshot instead of skipping the check; a disk in no pool is not
checked. Nothing is reserved: later writes can still fill a pool.

appsettings/tests/test_migrations.py

@@ -40,3 +40,14 @@ class NicTypeTestCase(TestCase):
         nic_type.fix_rtl8139(apps, SimpleNamespace(connection=connection))
         setting.refresh_from_db()
         self.assertEqual((setting.value, setting.choices), ("rtl8139", "default,e1000,e1000e,rtl8139,virtio"))
+
+
+class SnapshotFreeSpaceSettingsTestCase(TestCase):
+    def test_the_rows_fit_their_columns(self):
+        # PostgreSQL refuses a value longer than the column, sqlite does not
+        for key in ("SNAPSHOT_FREE_SPACE_PERCENT", "SNAPSHOT_FREE_SPACE_MIN_GIB"):
+            setting = AppSettings.objects.get(key=key)
+            for field in ("name", "key", "value", "description"):
+                limit = AppSettings._meta.get_field(field).max_length
+                if limit:
+                    self.assertLessEqual(len(getattr(setting, field)), limit, f"{key} {field}")