OvmfPkg/PlatformBootManagerLib: switch to QemuLoadImageLib

OvmfPkg/PlatformBootManagerLib: switch to QemuLoadImageLib

Replace the open coded sequence to load Linux on x86 with a short and
generic sequence invoking QemuLoadImageLib, which can be provided by
a generic version that only supports the LoadImage and StartImage boot
services, and one that incorporates the entire legacy loading sequence
as well.

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>