qemu: snapshot: Prevent too-nested domain XML when doing inactive snapshot

Similarly to 510d154a we need to prevent
doing too deeply nested backing chains and reject them with a sane error
message.

Add a loop to go through the snapshots prior to attempting actually
creating them to prevent some possible inconsistent scenarios.

We don't need to do it when reusing backing chains as we'll be
re-detecting the backing chain in that case anyways.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_driver.c

@@ -14653,6 +14653,9 @@ qemuDomainSnapshotCreateInactiveExternal(virQEMUDriverPtr driver,
         if (!snapdisk->src->format)
             snapdisk->src->format = VIR_STORAGE_FILE_QCOW2;
 
+        if (qemuDomainStorageSourceValidateDepth(defdisk->src, 1, defdisk->dst) < 0)
+            return -1;
+
         /* creates cmd line args: qemu-img create -f qcow2 -o */
         if (!(cmd = virCommandNewArgList(qemuImgPath,
                                          "create",