virtual: share buffer with client

virtual: share buffer with client

 - attach buffer
 - commit buffer
 - commit done

sequence :
 output commit -> backend call voutput commit func(not output commit handler this time) ->
 execute voutput attach buffer and commit buffer from voutput commit func(send buffer to client) ->
 commit done by client -> backend receive commit done so execute output commit handler

Change-Id: Ie0dde85f016ead14153c12e71d0d0ae6c2a65459
Signed-off-by: Junkyeong Kim <jk0430.kim@samsung.com>