OvmfPkg: plug DxeTpm2MeasureBootLib into SecurityStubDxe

OvmfPkg: plug DxeTpm2MeasureBootLib into SecurityStubDxe

The library registers a security management handler, to measure images
that are not measure in PEI phase. For example with the qemu PXE rom:

Loading driver at 0x0003E6C2000 EntryPoint=0x0003E6C9076 8086100e.efi

And the following binary_bios_measurements log entry seems to be
added:

PCR: 2	type: EV_EFI_BOOT_SERVICES_DRIVER     	size: 0x4e	digest: 70a22475e9f18806d2ed9193b48d80d26779d9a4

The following order of operations ensures that 3rd party UEFI modules,
such as PCI option ROMs and other modules possibly loaded from outside
of firmware volumes, are measured into the TPM:

(1) Tcg2Dxe is included in DXEFV, therefore it produces the TCG2
    protocol sometime in the DXE phase (assuming a TPM2 chip is present,
    reported via PcdTpmInstanceGuid).

(2) The DXE core finds that no more drivers are left to dispatch from
    DXEFV, and we enter the BDS phase.

(3) OVMF's PlatformBootManagerLib connects all PCI root bridges
    non-recursively, producing PciIo instances and discovering PCI
    oproms.

(4) The dispatching of images that don't originate from FVs is deferred
    at this point, by
    "MdeModulePkg/Universal/SecurityStubDxe/Defer3rdPartyImageLoad.c".

(5) OVMF's PlatformBootManagerLib signals EndOfDxe.

(6) OVMF's PlatformBootManagerLib calls
    EfiBootManagerDispatchDeferredImages() -- the images deferred in
    step (4) are now dispatched.

(7) Image dispatch invokes the Security / Security2 Arch protocols
    (produced by SecurityStubDxe). In this patch, we hook
    DxeTpm2MeasureBootLib into SecurityStubDxe, therefore image dispatch
    will try to locate the TCG2 protocol, and measure the image into the
    TPM2 chip with the protocol. Because of step (1), the TCG2 protocol
    will always be found and used (assuming a TPM2 chip is present).

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Stefan Berger <stefanb@linux.vnet.ibm.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>