xen/blkback: Convert to bdev_open_by_dev()

Convert xen/blkback to use bdev_open_by_dev() and pass the
handle around.

CC: xen-devel@lists.xenproject.org
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230927093442.25915-7-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>