Revert "OvmfPkg: use generic QEMU image loader for secure boot enabled ..."

Revert "OvmfPkg: use generic QEMU image loader for secure boot enabled ..."

This reverts commit ced77332cab626f35fbdb36630be27303d289d79.

The command

  virt-install --location NETWORK-URL

downloads the vmlinuz and initrd files from the remote OS tree, and passes
them to the guest firmware via fw_cfg.

When used with IA32 / X64 guests, virt-install expects the guest firmware
to do two things, at the same time:

- launch the fw_cfg kernel image even if the latter does not pass SB
  verification (SB checking is supposed to be bypassed entirely in favor
  of the Linux/x86 Boot Protocol),

- still let the guest kernel perceive SB as enabled.

Commit ced77332cab6 prevented this, by removing the Linux/x86 Boot
Protocol from such an OVMF image that was built with SECURE_BOOT_ENALBE.
While that's the right thing in theory, in practice "virt-install
--location NETWORK-URL" is entrenched, and we shouldn't break it.

We can tolerate the Linux/x86 Boot Protocol as a one-of-a-kind SB bypass
for direct-booted kernels, because:

- the fw_cfg content comes from QEMU, and the guest is already at QEMU's
  mercy,

- in the guest, OS boots after the initial installation will use "shim"
  rather than an fw_cfg kernel, which we can consider somewhat similar to
  "Audit Mode / Deployed Mode" (~ trust for install, lock down after).

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Message-Id: <20200615144514.24597-1-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[lersek@redhat.com: truncate the subject line, originally auto-generated
 by git-revert, to pacify PatchCheck.py]