- call mapiodev() with sizeof(struct xdc/xyc) rather than ra_len (which is

 - call mapiodev() with sizeof(struct xdc/xyc) rather than ra_len (which is
	zero since xdcmatch/xycmatch no longer sets it).   [if you call
	mapiodev() with a zero size it will reuse the KVA it returns, swiping
	the device out from under you!]
 - make xdc/xyc->iopbase point in the kernels DVMA space rather than at the
	normal malloc'd KVA.   this isn't compatable with sun4m [doesn't
	have a kernel DVMA space] and will need to be changed later (XXX).
 - move disk_attach() to before reading the disk label as per Jason.
	otherwise we are reading into an unallocated buffer (oops!)