block/curl: Use BDRV_SECTOR_SIZE

Currently, curl defines its own constant SECTOR_SIZE. There is no
advantage over using the global BDRV_SECTOR_SIZE, so drop it.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20161025025431.24714-2-mreitz@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
(cherry picked from commit 9054d9f6)

Conflicts:
	block/curl.c

* drop context dep on fffb6e12 / aio_bh_schedule_oneshot

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>