OvmfPkg/X86QemuLoadImageLib: handle EFI_ACCESS_DENIED from LoadImage()

OvmfPkg/X86QemuLoadImageLib: handle EFI_ACCESS_DENIED from LoadImage()

When an image fails Secure Boot validation, LoadImage() returns
EFI_SECURITY_VIOLATION if the platform policy is
DEFER_EXECUTE_ON_SECURITY_VIOLATION.

If the platform policy is DENY_EXECUTE_ON_SECURITY_VIOLATION, then
LoadImage() returns EFI_ACCESS_DENIED (and the image does not remain
loaded).

(Before <https://bugzilla.tianocore.org/show_bug.cgi?id=2129>, this
difference would be masked, as DxeImageVerificationLib would incorrectly
return EFI_SECURITY_VIOLATION for DENY_EXECUTE_ON_SECURITY_VIOLATION as
well.)

In X86QemuLoadImageLib, proceed to the legacy Linux/x86 Boot Protocol upon
seeing EFI_ACCESS_DENIED too.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2785
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200605235242.32442-1-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>