qemu-iotests: Improve portability by searching bash in the $PATH

Bash is not always installed as /bin/bash. In particular on OpenBSD,
the package installs it in /usr/local/bin.
Use the 'env' shebang to search bash in the $PATH.

Patch created mechanically by running:

  $ git grep -lE '#! ?/bin/bash' -- tests/qemu-iotests \
    | while read f; do \
      sed -i 's|^#!.\?/bin/bash$|#!/usr/bin/env bash|' $f; \
    done

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/tests/qemu-iotests/053 b/tests/qemu-iotests/053
index afa109c..50c62f0 100755
--- a/tests/qemu-iotests/053
+++ b/tests/qemu-iotests/053
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test qemu-img convert when image length is not a multiple of cluster size
 #