xen, blkback: check if ring has been initialized before accessing it

This commit lets the xen_update_blkif_status() function check if a
ring has already been initialized (as most of them are only on frontend
connection) before trying to dereference the structure.
xen, blkback: init list before using it

This commit lets the xen_blkif_ring_alloc() function correctly
initialize the list of pending requests for a ring before trying
to insert requests in it.
xen, blkback: add debugging prints and logic

This commit adds to the blkback driver some prints and logic useful for
the current debugging and testing activities.
drivers, block: add first version of fake multi-queue test driver

The driver added by this commit returns a fixed value for each of
the submit queues, so that consistency of access to hardware ctx
can be verified very easily. As of now, one only device is created
as soon as the module is loaded, and the number of hardware queues
equals the number of available CPUs.