qemu-trad: stop using the default IOREQ server

qemu-trad: stop using the default IOREQ server

Because qemu-trad is using the legacy HVM param mechanism of hooking into
Xen, it means that Xen has to maintain the notion of a 'default' IOREQ
server which is where all I/O goes if no other device model claims it.
Maintaining this code in Xen has a cost and it would be good if the project
no longer had to pay it.

This patch makes the necessary minimal changes to the qemu-trad to use the
IOREQ server API to hook into Xen. This means the default IOREQ server
will no longer be in use and thus it no longer needs to be maintained.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>