[illumos-Developer] webrev & design change: blkdev should support dump

[Garrett D'Amore]

As part of some some future support coming (virtio block device
support), I've added support for dump(9e) to blkdev.  This necessitated
some changes to the blkdev API, as it turns out that I need to be able
to *read* a label from a blkdev device even with interrupts disabled.

The webrev is here:

http://mexico.purplecow.org/gdamore/webrev/blkdevdump/

The API, which is Consolidation Private, has got the following changes: 

bd_xfer_t now has an x_flags field.  The only flag defined is
BD_XFER_POLL, which means that the request should be treated as a
"polled" request -- i.e. the driver must poll for completion rather than
relying on interrupts.  This can apply especially to the o_read and
o_write entry points.

The o_dump entry point is removed. It is now handled instead by using
BD_XFER_POLL.

I've tested the driver with SDcard, and we've tested these changes with
a new blkdev provider using virtio/kvm.  This will be coming out for
review very soon as well.

	- Garrett