block: Add SG_IO device check in refresh_total_sectors()

This patch adds a special case check for scsi-generic devices in
refresh_total_sectors() to skip the subsequent BlockDriver->bdrv_getlength()
that will be returning -ESPIPE from block/raw-posic.c:raw_getlength() for
BlockDriverState->sg=1 devices.

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