xen, blkback: move pool of pending requests to be per-device

This commit reverts one of the previous changes that moved pending
requests to be per-ring. This commit does not help with locking,
but as of now gets out of the way some memory issues that were long
ago expected by Konrad Wilk.
xen, blkback: support old names for xenblkd threads

This commit adds code to support the old naming convention for
xenblkd threads, which used only the interface name as the block
threads were per-device.
xen, blkback: factor out code to advertise number of hw queues

This commit factors out in a function the code used to advertise
the number of hardware queues available for a device, including
it in a XenBus transaction. This has no other purpose than
facilitating debugging of blkback code.
xen, blkback: add comment about retrieving ringrefs and evtchns

This commit adds a triple-X comment about the code retrieving
ring references and event channels. Even if the device supports
multiqueue, and the backend does, it is not sure that also the
guest's frontend supports multiqueue. Not being able to read
ring references and event channels with the new naming convention
does not therefore imply a failure. It will be necessary to add
code to fall back to searching for keys with the old names before
returning with an error.