OvmfPkg: use generic QEMU image loader for secure boot enabled builds

OvmfPkg: use generic QEMU image loader for secure boot enabled builds

The QemuLoadImageLib implementation we currently use for all OVMF
builds copies the behavior of the QEMU loader code that precedes it,
which is to disregard UEFI secure boot policies entirely when it comes
to loading kernel images that have been specified on the QEMU command
line. This behavior deviates from ArmVirtQemu based builds, which do
take UEFI secure boot policies into account, and refuse to load images
from the command line that cannot be authenticated.

The disparity was originally due to the fact that the QEMU command line
kernel loader did not use LoadImage and StartImage at all, but this
changed recently, and now, there are only a couple of reasons left to
stick with the legacy loader:
- it permits loading images that lack a valid PE/COFF header,
- it permits loading X64 kernels on IA32 firmware running on a X64
  capable system.

Since every non-authentic PE/COFF image can trivially be converted into
an image that lacks a valid PE/COFF header, the former case can simply
not be supported in a UEFI secure boot context. The latter case is highly
theoretical, given that one could easily switch to native X64 firmware in
a VM scenario.

That leaves us with little justification to use the legacy loader at all
when UEFI secure boot policies are in effect, so let's switch to the
generic loader for UEFI secure boot enabled builds.

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>