tools/rbd: add support of new device type for ubbd

tools/rbd: add support of new device type for ubbd

ubbd (Userspace Backend Block Device) is a method to build block device
and handle IO by process started in userspace. That means we can provide
a generic block device to user and handle IO requests by librbd
in linux.

This way, we can allow user to use rbd image as a linux block device
supporting full image features, especial the journaling feature, which
is not supported by krbd.

For more information: https://github.com/DataTravelGuide/ubbd

Signed-off-by: Dongsheng Yang <dongsheng.yang.linux@gmail.com>