tools/oxenstored: Bind the DOM_EXC VIRQ in in Event.init()

tools/oxenstored: Bind the DOM_EXC VIRQ in in Event.init()

Xenstored always needs to bind the DOM_EXC VIRQ.

Instead of doing it shortly after the call to Event.init(), do it in the
constructor directly.  This removes the need for the field to be a mutable
option.

It will also simplify a future change to support live update.  Rename the
field from virq_port (which could be any VIRQ) to it's proper name.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit 9804a5db435fe40c8ded8cf36c2d2b2281c56f1d)