Optimize OVMF for Xen HVM domains

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. We have also enabled Xen support in OVMF. This project is going to add Xen specific support in OVMF, such as PV (para-virtualization) front-end drivers, to improve boot and runtime performance. The PV drivers are to discourage the use of the emulated devices provided by Xen qemu-dm and make OVMF more Xen-aware. They will make OVMF boot on Xen quickly and gain high I/O performance.