xen/blkback: Add some comments.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --cc drivers/xen/blkback/blkback.c
Simple merge

diff --cc drivers/xen/blkback/common.h
Simple merge

diff --cc drivers/xen/blkback/interface.c
Simple merge

diff --cc drivers/xen/blkback/vbd.c
Simple merge

diff --cc drivers/xen/blkback/xenbus.c
Simple merge