block: vs_block_server: Fix bioset allocation

Add missing bioset allocation while initializing the virtual server
struct.

Change-Id: If76290a2191e5000b5bcdfff815227a9027d2a9c
Acked-by: Tom Kuo <tkuo@qti.qualcomm.com>
Signed-off-by: Chris Lew <clew@codeaurora.org>