iscsi: fix assertion in is_sector_request_lun_aligned

Commit 94d047a added an assertion the the request alignment check.
This introduced 2 issues:
 a) A off-by-one error since a request of BDRV_REQUEST_MAX_SECTORS
    is actually allowed.
 b) The bdrv_get_block_status call in the read path to check the allocation
    status requests up to INT_MAX sectors which triggers the assertion.

Fixes: 94d047a35bf663e28f8fef137544d8ea78165add
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1466414680-18383-1-git-send-email-pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
