ide: commonize io_buffer_index initialization

ide: commonize io_buffer_index initialization

Resetting the io_buffer_index to 0 is commonized,
with the exception of the case within ide_atapi_cmd_reply,
where we need to reset this index to 0 prior to the
ide_atapi_cmd_reply_end call.

Note that not all calls to ide_atapi_cmd_reply_end
expect the index to be 0, so setting it there is
not appropriate.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1424708286-16483-12-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>