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 <[email protected]>
Signed-off-by: John Snow <[email protected]>
Message-id: 1424708286[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>