[Qemu-block] How to specify the full block driver tree on the CLI ?

[Daniel P. Berrange]

As previously mentioned, I'm working on support for LUKS full disk
encryption in QEMU. I have a simple driver implemented that works
on top of plain files. eg I can launch qemu-io thus:

 $ qemu-io /home/berrange/VirtualMachines/demo.luks-aes-cbc-plain-sha256

and it'll probe the luks format & instantiate my "luks" block driver impl
on top of the "file" driver. IIUC, I should be able to layer this format
driver on top of any of the QEMU block driver backends though. In particular
I want to be able to layer it on top of any of the network drivers (RBD,
iSCSI and glusterfs). I'm struggling to figure out the right syntax to
specify this to QEMU though, using either qemu-io, or the system emulators
with the -drive arg.  Are there any docs somewhere about the way to
structure the command line arguments to build up a stack of block drivers.
I'd like to figure out the following combinations, for qemu-io, qemu-img
and system emulator -drive syntax.

 - luks -> file
 - qcow2 -> luks -> file
 - luks -> rbd
 - luks -> iscsi
 - luks -> glusterfs

Currently the only required QemuOpt for the luks driver is the ID of
a secret to provide the password.

Regards,
Daniel
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