partially fix #4012: migrate: mention snapshots referencing a local ISO image

When a VM has no local ISO image attached anymore, but one of its
snapshots still references it, migration fails with:

  can't migrate local disk 'local:iso/debian.iso': local cdrom image

which does not tell where the image is coming from, since it is not
visible in the current configuration of the VM.

Add the names of the snapshots referencing the image to the error in
that case, like it is already done for a physical CD/DVD drive. The
check for the physical drive now reuses the same hint, which also
uses the plural form if more than one snapshot references it.

Add a migration test with an ISO image on the 'local' storage that is
only referenced in a snapshot. This requires the 'iso' content type
for the 'local' storage in the test.

Signed-off-by: Michal Fox <me@dualfroz.com>
Link: https://lore.proxmox.com/20261008094650.7-3-me@dualfroz.com
[FE: note that it's a partial fix as the UI still needs to be improved]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

src/test/run_qemu_migrate_tests.pl

@@ -37,6 +37,7 @@ my $storage_config = {
         local => {
             content => {
                 images => 1,
+                iso => 1,
             },
             path => "/var/lib/vz",
             type => "dir",
@@ -438,7 +439,18 @@ my $source_vdisks = {
 
 # Non-image volumes on the source, e.g. ISO images, with the same structure as $source_vdisks. They
 # are only used for the mocked volume_size_info(), because vdisk_list() only returns images.
-my $source_other_volumes = {};
+my $source_other_volumes = {
+    'local' => [
+        {
+            'ctime' => 1589439681,
+            'format' => 'iso',
+            'parent' => undef,
+            'size' => 663748608,
+            'used' => 663748608,
+            'volid' => 'local:iso/debian.iso',
+        },
+    ],
+};
 
 my $default_expected_calls_online = {
     move_config_to_node => 1,
@@ -1240,6 +1252,41 @@ my $tests = [
             },
         },
     },
+    {
+        name => '105_local_iso_in_snapshot',
+        target => 'pve1',
+        vmid => 105,
+        vm_status => {
+            running => 0,
+        },
+        config_patch => {
+            snapshots => {
+                ohsnap => {
+                    ide2 => 'local:iso/debian.iso,media=cdrom',
+                },
+            },
+        },
+        expected_calls => {},
+        expect_die => "can't migrate local disk 'local:iso/debian.iso': local cdrom image"
+            . " (referenced in snapshot - ohsnap)",
+        expected => {
+            source_volids => local_volids_for_vm(105),
+            target_volids => {},
+            vm_config => get_patched_config(
+                105,
+                {
+                    snapshots => {
+                        ohsnap => {
+                            ide2 => 'local:iso/debian.iso,media=cdrom',
+                        },
+                    },
+                },
+            ),
+            vm_status => {
+                running => 0,
+            },
+        },
+    },
     {
         name => '105_cdrom',
         target => 'pve1',