OvmfPkg/OvmfXen: use a TimerLib instance that depends only on the CPU

OvmfPkg/OvmfXen: use a TimerLib instance that depends only on the CPU

The ACPI Timer isn't present in a PVH guest, but local APIC works on
both PVH and HVM.

Note that the use of SecPeiDxeTimerLibCpu might be an issue with a
driver of type DXE_RUNTIME_DRIVER. I've attempted to find out which of
the DXE_RUNTIME_DRIVER uses the TimerLib at runtime. I've done that by
replacing the TimerLib evaluation in
[LibraryClasses.common.DXE_RUNTIME_DRIVER] by a different one and
checking every module that uses it (with the --report-file=report
build option).

ResetSystemRuntimeDxe is calling the TimerLib API at runtime to do the
operation "EfiResetCold", so this may never complete if the OS have
disabled the Local APIC Timer.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-10-anthony.perard@citrix.com>