rbd: Use qemu-img to convert from QCOW2 templates to RBD directly.

This saves the step of writing to a temporary image in /tmp first before
writing to RBD.

This is possible due to a new version in librbd. With the rbd_default_format
setting we can now force qemu-img to create format 2 RBD images.

This is available since Ceph version 0.67.5 (Dumpling).
rbd: Use qemu-img to backup up a RBD snapshot to Secondary Storage

This reduces the amount of time and storage it takes dramatically. We no longer
do a full copy, but a sparse copy. The destination image is still in RAW
format, but we only copy over used blocks.

Qemu is also better in doing this then us doing it in Java code.