ahci: Fix byte count regression for ATAPI/PIO

ahci: Fix byte count regression for ATAPI/PIO

This patch fixes a regression caused by commit
659142ecf71a0da240ab0ff7cf929ee25c32b9bc.
The problem occurs when we wish to return early
from the ahci_start_transfer function, but are now
updating the transferred byte count in the AHCI
command header via ahci_commit_buf.

This will cause problems in the Windows 8 installer.

Don't update the byte count in the command header
for the transmission of ATAPI packets: These commands
will distort the final byte count of the actual data
payload.

The call to ahci_commit_buf remains in the "out"
portion of the call in order to clean up the sglist.
The byte count is maintained by forcing size to be 0.

Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>