tests/qemu-iotests: Do not hard-code the path to bash

bash is installed in a different directory on non-Linux systems like
FreeBSD. Do not hard-code /bin/bash here so that the tests can run
there, too.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20190502084506.8009-4-thuth@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>

tests/qemu-iotests/239

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test case for dmg
 #

tests/qemu-iotests/240

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test hot plugging and unplugging with iothreads
 #

tests/qemu-iotests/241

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test qemu-nbd vs. unaligned images
 #

tests/qemu-iotests/243

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test qcow2 preallocation
 #

tests/qemu-iotests/244

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test qcow2 with external data files
 #