tests/qemu-iotests: Use GNU sed in two more spots where it is necessary

These two spots have been missed in commit 9086c763 ("Rework the
checks and spots using GNU sed") - they need GNU sed, too, since they
are using the "+" address form.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220309101626.637836-1-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>