xen, blkfront: base changes to separate request and response rings

This commit is the counterpart of blkback's 6e4f926. It separates
from the main per-ring structure the fields that are instrumental
to handling the ringbuffer and locking it.
Please NOTE that this is work in progress, and is still undergoing
major bugs affecting the correct functioning of the driver. Also,
the commit is very noisy and the code not clean as it is undergoing
heavy debugging: this commit is just for documentation.