qemu/rbd: improve rbd device specification

qemu/rbd: improve rbd device specification

This improves the support for qemu rbd devices by adding support for a few
key features (e.g., authentication) and cleaning up the way in which
rbd configuration options are passed to qemu.

An <auth> member of the disk source xml specifies how librbd should
authenticate. The username attribute is the Ceph/RBD user to authenticate as.
The usage or uuid attributes specify which secret to use. Usage is an
arbitrary identifier local to libvirt.

The old RBD support relied on setting an environment variable to
communicate information to qemu/librbd.  Instead, pass those options
explicitly to qemu.  Update the qemu argument parsing and tests
accordingly.

Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>