[libvirt] Block pull API QEMU progress update

[Stefan Hajnoczi]

I am sending this email as an update on the new image streaming for
QEMU (aka block pull in libvirt).

Adam's libvirt virDomainBlockPull patches were recently merged to
allow populating a disk image while the domain is running.  The QEMU
side implementation is not merged into qemu.git yet because similarity
with the live block copy feature has sparked discussion on how to
unify the two.

The QEMU APIs for image streaming and live block copy may or may not
be changed.  I think they should stay as they are because they provide
good interfaces for those two features.  However, it is possible that
unifying the implementations of the two features internally
necessitates changes to the QEMU APIs.

If changes are made, the question becomes whether they require
changing the libvirt block pull (and proposed live block copy) API.

Right now there is no proposal to change the QEMU APIs for the two
features but I think it is useful for libvirt to be aware of the
status on these features.  Perhaps a precautionary measure like not
committing to the APIs until the QEMU patches are merged is
worthwhile, just for safety.

Marcelo, Kevin, and I are working on unifying the QEMU side
implementations of live block copy and image streaming.  Hopefully the
specifics will be agreed soon and we can then enable these features in
QEMU and libvirt.

Stefan