tests: fix cdrom_pio_impl in ide-test

The check for the cleared BSY flag has to be performed
before each data transfer and not just before the
first one.

Commit 5f81724d revealed this glitch as the BSY flag
was not set in ATAPI PIO transfers before.

While at it fix the descriptions and add a comment before
the nested for loop that transfers the data.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1448029742-19771-1-git-send-email-pl@kamp.de
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
