OvmfPkg/OvmfXen: use RealTimeClockRuntimeDxe from EmbeddedPkg

OvmfPkg/OvmfXen: use RealTimeClockRuntimeDxe from EmbeddedPkg

A Xen PVH guest doesn't have a RTC that OVMF would expect, so
PcatRealTimeClockRuntimeDxe fails to initialize and prevent the
firmware from finish to boot. To prevent that, we will use
XenRealTimeClockLib which simply always return the same time.
This will work on both Xen PVH and HVM guests.

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: <20190813113119.14804-36-anthony.perard@citrix.com>