rbd: add --xio option for xio_connect

Signed-off-by: Casey Bodley <casey@linuxbox.com>
librados: add rados_xio_connect() to c api

Signed-off-by: Casey Bodley <casey@linuxbox.com>
librbd: install rbd headers

Signed-off-by: Casey Bodley <casey@linuxbox.com>
Added mkcephfs for an email patch