block: Make find_image_format() return 'raw' BlockDriver for SG_IO devices

This patch adds a special BlockDriverState->sg check in block.c:find_image_format()
after bdrv_file_open() -> block/raw-posix.c:hdev_open() has been called to determine
if we are dealing with a Linux host scsi-generic device.

The patch then returns the BlockDriver * from bdrv_find_format("raw"), skipping the
subsequent bdrv_read() and rest of find_image_format().

Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
