xen, blkback: support multiple rings

This commit introduces actual support for multiple rings, in case
the underlying device is multi-queue-capable.
xen, blkback: use per-CPU rings even if the device is not mq-capable

This commits always sets the number of allocated rings to the number
of CPUs of the driver domain, therefore letting the I/O requests
be processed by multiple threads even if the device is not multi-
queue-capable.

Signed-off-by: Bob Liu <bob.liu@oracle.com>