Working patchset Ceph/RBD volume support

[gxben]

@dmacvicar I do have working patchset for minimalistic Ceph/RBD support for distributed volumes usage.

As Ceph/RBD configuration can be quite complex and exhaustive, XSLT could be considered but is quite a pain to offer.

Alternatively, my patch uses the following assumptions:

• no pool creation/deletion support, we only care about volumes
• pool configuration is left to libvirt's sysadmin, which is responsible for designing its XML configuration accordingly and uses virsh pool-define to set it up correctly
• there will be a few rbd specific schema updates (addition, no breaking changes) to properly declare the rbd pool/host/port to be used.
• volume creation and remote upload is not fully support by libvirt as of now (or so it seems), so my alternative proposal is to ensure we have a working clone for volume creation, i.e. use an already existing base_volume (up to libvirt's sysadmin to have it deployed beforehand, as a cloudimg base template or whatever the name) as source.

Is it ok with you if I submit a pull request for that (as much as possible I'd avoid keeping it as a fork) ?