OvmfPkg: implement QEMU loader library for X86 with legacy fallback

OvmfPkg: implement QEMU loader library for X86 with legacy fallback

Implement another version of QemuLoadImageLib that uses LoadImage and
StartImage, but falls back to the legacy Linux loader code if that
fails. The logic in the legacy fallback routines is identical to the
current QEMU linux loader for X64 and IA32.

Note the use of the OVMF_LOADED_X86_LINUX_KERNEL protocol for the legacy
loaded image: this makes it possible to expose the LoadImage/StartImage
abstraction for the legacy loader, using the EFI paradigm of identifying
a loaded image solely by a handle.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2566
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>