ide: Update ide_drive_get to be HBA agnostic

ide: Update ide_drive_get to be HBA agnostic

Instead of duplicating the logic for the if_ide
(bus,unit) mappings, rely on the blockdev layer
for managing those mappings for us, and use the
drive_get_by_index call instead.

This allows ide_drive_get to work for AHCI HBAs
as well, and can be used in the Q35 initialization.

Lastly, change the nature of the argument to
ide_drive_get so that represents the number of
total drives we can support, and not the total
number of buses. This will prevent array overflows
if the units-per-default-bus property ever needs
to be adjusted for compatibility reasons.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Message-id: 1412187569[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>