ide-test: cdrom_pio_impl fixup

Final tidying: move the interrupt wait into the loop,
document that the status read clears the IRQ, and move
the final interrupt check outside of the loop.

This should be functionally equivalent to how it works
currently, but a little less ambiguous and slightly more
explicit about the state transitions.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1448060035-31973-3-git-send-email-jsnow@redhat.com
