ArmVirtPkg: reinstate timer unmask quirk for Xen

Commit 411a373ed6 ("ArmPkg/TimerDxe: remove workaround for KVM timer
handling") removed the virtual timer handling quirk that cleared the
mask bit in the control register when enabling the timer, under the
assumption that only ancient KVM host implementations required it.
However, Julien reports that Xen also masks the timer interrupt in the
guest view of the timer control register, and therefore needs the same
quirk.

So let's reinstate it, but using a Xen specific implementation of the
timer support library, so that other virt platforms remain unchanged.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>

ArmVirtPkg/Library/XenArmGenericTimerVirtCounterLib/XenArmGenericTimerVirtCounterLib.inf

@@ -0,0 +1,33 @@
+#/** @file
+#  Implement ArmGenericTimerCounterLib for Xen using the virtual timer
+#
+#  Copyright (c) 2014 - 2018, Linaro Ltd. All rights reserved.<BR>
+#
+#  This program and the accompanying materials
+#  are licensed and made available under the terms and conditions of the BSD License
+#  which accompanies this distribution. The full text of the license may be found at
+#  http://opensource.org/licenses/bsd-license.php
+#  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+#
+#
+#**/
+
+[Defines]
+  INF_VERSION                    = 0x0001001A
+  BASE_NAME                      = XenArmGenericTimerVirtCounterLib
+  FILE_GUID                      = e3913319-96ac-4ac0-808b-8edb8776a51c
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = ArmGenericTimerCounterLib
+
+[Sources]
+  XenArmGenericTimerVirtCounterLib.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  ArmPkg/ArmPkg.dec
+
+[LibraryClasses]
+  ArmLib
+  BaseLib