block: use bdrv_is_sg() helper instead of raw bs->sg reading

block: use bdrv_is_sg() helper instead of raw bs->sg reading

I believe that if the helper exists, it must be used always for reading
of the value. It breaks expectations in the other case.

Signed-off-by: Denis V. Lunev <[email protected]>
CC: Kevin Wolf <[email protected]>
CC: Hanna Reitz <[email protected]>
CC: Stefan Hajnoczi <[email protected]>
CC: Fam Zheng <[email protected]>
CC: Ronnie Sahlberg <[email protected]>
CC: Paolo Bonzini <[email protected]>
CC: Peter Lieven <[email protected]>
CC: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20220817083736[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>