xen, blkback: sanitize memory after rings allocation

The krealloc() function does not zero memory after allocation. This
commit adds an invocation of memset() to be sure that it's sanitized.
xen, blkback: use correct device when binding evtchn irqhandler

Previously, irq and evtchn-related information was kept in the xen_blkif
structure. Now, it has been moved to the xen_blkif_ring. Use the latter
structure when binding the evtchn.
xen, blkback: use correct indexes for the loop in xen_blkif_free()

This commit uses the correct local variables in the per-ring loop
executed by the xen_blkif_free() function. This fixes a WARN which
was trigger in said function which previously messed up indexes.
xen, blkback: remove debug printks and do minor cleanup

This commit cleans up blkback code in preparation for the next
steps. This involves removing debug printks and using local
variables when necessary instead of dereferencing pointers
multiple times.