OvmfPkg/SmmControl2Dxe: select broadcast SMI if available

OvmfPkg/SmmControl2Dxe: select broadcast SMI if available

When writing to IO port 0xB2 (ICH9_APM_CNT), QEMU by default injects an
SMI only on the VCPU that is writing the port. This has exposed corner
cases and strange behavior with edk2 code, which generally expects a
software SMI to affect all CPUs at once. We've experienced instability
despite the fact that OVMF sets PcdCpuSmmApSyncTimeout and
PcdCpuSmmSyncMode differently from the UefiCpuPkg defaults, such that they
match QEMU's unicast SMIs better. (Refer to edk2 commits 9b1e378811ff and
bb0f18b0bce6.)

Using the new fw_cfg-based SMI feature negotiation in QEMU (see commits
50de920b372b "hw/isa/lpc_ich9: add SMI feature negotiation via fw_cfg" and
5ce45c7a2b15 "hw/isa/lpc_ich9: add broadcast SMI feature"), we can ask
QEMU to broadcast SMIs. Extensive testing from earlier proves that
broadcast SMIs are only reliable if we use the UefiCpuPkg defaults for the
above PCDs. With those settings however, the broadcast is very reliable --
the most reliable configuration encountered thus far.

Therefore negotiate broadcast SMIs with QEMU, and if the negotiation is
successful, dynamically revert the PCDs to the UefiCpuPkg defaults.

Setting the PCDs in this module is safe:

- only PiSmmCpuDxeSmm consumes them,

- PiSmmCpuDxeSmm is a DXE_SMM_DRIVER, launched by the SMM_CORE
  (MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf),

- the SMM_CORE is launched by the SMM IPL runtime DXE driver
  (MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf),

- the SMM IPL has a DEPEX on EFI_SMM_CONTROL2_PROTOCOL,

- OvmfPkg/SmmControl2Dxe produces that protocol.

The end result is that PiSmmCpuDxeSmm cannot be dispatched before
SmmControl2Dxe installs EFI_SMM_CONTROL2_PROTOCOL and returns from its
entry point. Hence we can set the PCD's consumed by PiSmmCpuDxeSmm in
SmmControl2Dxe.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=230
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>