xen-disk: use an IOThread per instance

This patch allocates an IOThread object for each xen_disk instance and
sets the AIO context appropriately on connect. This allows processing
of I/O to proceed in parallel.

The patch also adds tracepoints into xen_disk to make it possible to
follow the state transtions of an instance in the log.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>