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

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

Similarly to 510d154a0b41aa70aadabc0918d16dee22882394 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: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>