xen: allow to register event channels without handlers

This is needed by the event channel user-space device, that requires
registering event channels without unmasking them. intr_add_handler
will unconditionally unmask the event channel, so we avoid calling it
if no filter/handler is provided, and then the user will be in charge
of calling it when ready.

In order to do this, we need to change the opaque type
xen_intr_handle_t to contain the event channel port instead of the
opaque cookie returned by intr_add_handler, since now registration of
event channels without handlers are allowed. The cookie will now be
stored inside of the private xenisrc struct. Also, introduce a new
function called xen_intr_add_handler that allows adding a
filter/handler after the event channel has been registered.

Sponsored by: Citrix Systems R&D

x86/xen/xen_intr.c:

• Leave the event channel without a handler if no filter/handler is provided to xen_intr_bind_isrc.
• Don't perform an evtchn_mask_port, intr_add_handler will already do it.
• Change the opaque type xen_intr_handle_t to contain a pointer to the event channel port number, and make the necessary changes to related functions.
• Introduce a new function called xen_intr_add_handler that can be used to add filter/handlers to an event channel after registration.

xen/xen_intr.h:

• Add prototype of xen_intr_add_handler.