OvmfPkg/SmbiosPlatformDxe: return EFI_NOT_FOUND if there is no SMBIOS data
    
    According to the function-top comment, SmbiosTablePublishEntry() is
    supposed to return an error code if no SMBIOS data is found, from either
    GetXenSmbiosTables() or GetQemuSmbiosTables(). Currently the function
    returns EFI_SUCCESS in this case however (propagated from
    gBS->LocateProtocol()). Make the return code match the documentation.
    
    (This issue is not too important, but it gets in the way of splitting the
    entry point function next.)
    
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>