The attached fixes a few things to make the blkfront and netfront

The attached fixes a few things to make the blkfront and netfront
drivers better follow the Linux driver model.  Changes include:
* Rename xenbus_register functions to more closely match what's used
with other buses.  Adjust callers
* There's no need to wait for the first vbd of vif and do a timeout if
they fail, device probing is async
* Add sysfs attributes for examining type and nodename
* Set device of gendisk so /sys/block/foo/device points to the right
place
* Set device of net_device so that /sys/class/net/foo/device points to
the right place

Signed-off-by: Jeremy Katz <katzj@redhat.com>