xen: Remove event channel notification through Xen PCI platform device

Ever since commit 254d1a3f02eb ("xen/pv-on-hvm kexec: shutdown watches
from old kernel") using the INTx interrupt from Xen PCI platform
device for event channel notification would just lockup the guest
during bootup.  postcore_initcall now calls xs_reset_watches which
will eventually try to read a value from XenStore and will get stuck
on read_reply at XenBus forever since the platform driver is not
probed yet and its INTx interrupt handler is not registered yet. That
means that the guest can not be notified at this moment of any pending
event channels and none of the per-event handlers will ever be invoked
(including the XenStore one) and the reply will never be picked up by
the kernel.

The exact stack where things get stuck during xenbus_init:

-xenbus_init
 -xs_init
  -xs_reset_watches
   -xenbus_scanf
    -xenbus_read
     -xs_single
      -xs_single
       -xs_talkv

Vector callbacks have always been the favourite event notification
mechanism since their introduction in commit 38e20b07efd5 ("x86/xen:
event channels delivery on HVM.") and the vector callback feature has
always been advertised for quite some time by Xen that's why INTx was
broken for several years now without impacting anyone.

Luckily this also means that event channel notification through INTx
is basically dead-code which can be safely removed without impacting
anybody since it has been effectively disabled for more than 4 years
with nobody complaining about it (at least as far as I'm aware of).

This commit removes event channel notification through Xen PCI
platform device.

Cc: Boris Ostrovsky <[email protected]>
Cc: David Vrabel <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: [email protected]
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Stefano Stabellini <[email protected]>
Cc: Julien Grall <[email protected]>
Cc: Vitaly Kuznetsov <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Ross Lagerwall <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Anthony Liguori <[email protected]>
Signed-off-by: KarimAllah Ahmed <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Signed-off-by: David Vrabel <[email protected]>

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