Accessing Ceph (rbd), sheepdog, etc using libguestfs

Thanks to infernix who contributed this tip on how to use libguestfs to access Ceph (and in theory, sheepdog, gluster, iscsi and more) devices.

If you apply this small patch to libguestfs you can use these distributed filesystems straight away by doing:

$ guestfish
><fs> set-attach-method appliance
><fs> add-drive /dev/null
><fs> config -set drive.hd0.file=rbd:pool/volume
><fs> run

… followed by usual guestfish commands.

This is a temporary hack, until we properly model Ceph (etc) through the libguestfs stable API. Nevertheless it works as follows:

1. The add-drive /dev/null adds a drive, known to libguestfs.
2. Implicitly this means that libguestfs adds a -drive option when it runs qemu.
3. The custom qemu -set drive.hd0.file=... parameter modifies the preceding -drive option added by libguestfs so that the file is changed from /dev/null to whatever you want. In this case, to a Ceph rbd:... path.

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