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

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 <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>