ioreq-server: on-demand creation of ioreq server

ioreq-server: on-demand creation of ioreq server

This patch only creates the ioreq server when the legacy HVM parameters
are read (by an emulator).

A lock is introduced to protect access to the ioreq server by multiple
emulator/tool invocations should such an eventuality arise. The guest is
protected by creation of the ioreq server only being done whilst the
domain is paused.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>