Enable Xen Support in OVMF

by Bei Guan for Tianocore

Open Virtual Machine Firmware (OVMF) is an implementation of Unified Extensible Firmware Interface (UEFI) and it contains a sample UEFI firmware for QEMU and KVM. This project will enable Xen Support in OVMF. With OVMF firmware, a UEFI-compliant OS can boot from the Xen virtualization platform. Meanwhile, UEFI drivers and applications can run on Xen. This project also includes some additional work to optimize OVMF on Xen.