Blkfront greater-than-16-vbd fixes

Blkfront greater-than-16-vbd fixes

While reading through the code again, I realized that if you did
something like:

block-attach xvdb
block-attach xvdq
block-attach xvdc

The extended blkfront code would allocate the wrong device for xvdc,
since we had changed mi->type along the way.  To fix this, have a
separate "major_info" entry for the extended block devices, even
though it isn't technically a different major number.  With these
fixes in place, the above sequence works for me.

Signed-off-by: Chris Lalancette <clalance@redhat.com>