block: expect get_block_status errors in bdrv_make_zero

during testing around with 4k LUNs a bad target implementation
triggert an -EIO in iscsi_get_block_status, but it got never caught
resulting in an infinite loop.

CC: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
