add async backup/restore to rbd backup
add async backup/restore to rbd backup

User can use --async to put backup/restore background.

TODO: add query_progress API to report background op status.

eoncloud-dev/eontools,ZhangZhenhua/eontools,ZhangZhenhua/eontools,eoncloud-dev/eontools,ZhangZhenhua/eontools,eoncloud-dev/eontools