[libvirt] [RFC PATCH 0/5] Allow data upload/download to/from storage volumes

This patch series introduces two new APIs to allow data upload/download to/from storage volumes, using the streams APIs. Mostly I wrote this so I had an easier way to test the streams handling in my RPC patch series than using QEMU tunnelled migration code. Cole has expressed an interest in this kind of functionality in the past to allow virt-manager to upload kernel/initrd to remote hosts for provisioning guests. Most of the code here is pretty boring & trivial. The first patch is the one that is remotely interesting. One might argue we should just spawn 'dd' instead of this, but dd has some annoying aspects (you can't control its write offset in units smaller than the block size) and this could be useful more broadly.