ArmVirtPkg: reinstate timer unmask quirk for Xen

ArmVirtPkg: reinstate timer unmask quirk for Xen

Commit 411a373ed642 ("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>