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

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: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20190502084506.8009-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>