OvmfPkg/OvmfXen: Introduce XenTimerDxe

OvmfPkg/OvmfXen: Introduce XenTimerDxe

"OvmfPkg/8254TimerDxe" is replaced with a Xen-specific
EFI_TIMER_ARCH_PROTOCOL implementation. Also remove
8259InterruptControllerDxe as it is not used anymore.

This Timer uses the local APIC timer as time source as it can work on
both a Xen PVH guest and an HVM one.

Based on the "OvmfPkg/8254TimerDxe" implementation.

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: <20190813113119.14804-31-anthony.perard@citrix.com>