layer asynchronous, so keeping the sync/async boundary within aoedev.c
seems cleanest for now.

async_wait(), though we may wish to move this higher up the stack, and
consider making the block device model asynchronous.  (There is only a
marginal cost for synchronous devices, since they can simply call
async_done() before returning; async_wait() will work seamlessly in this
situation).

Removed data_in_len and data_out_len from ata_command structure; the
lengths are implied by the sector count and the presence of the data_in
or data_out pointers.

Changed AoE code to use subcommands by default, and made aoe_issue()
nonblocking (with completion via async_wait()).

Increase code simplicity at the expense of around 64 bytes.

enough to support both real IDE chipsets and AoE.

READ(16) is mandatory); we must use READ CAPACITY (10) first and then
use READ CAPACITY (16) if the READ CAPACITY (10) returns "out of range".

the kernel), which encapsulates the information needed to refer to an
external buffer.  Under normal operation, this can just be a void *
equivalent, but under -DKEEP_IT_REAL it would be a segoff_t equivalent.

Use this concept to avoid the need for bounce buffers in int13.c,
which reduces memory usage and opens up the possibility of using
multi-sector reads.

Extend the block-device API and the SCSI block device implementation
to support multi-sector reads.

Update iscsi.c to use user buffers.

Move the obsolete portions of realmode.h to old_realmode.h.

MS-DOS now boots an order of magnitude faster over iSCSI (~10 seconds
from power-up to C:> prompt in bochs).

implementation.