OvmfPkg/ResetSystemLib: introduce the DxeResetSystemLib instance

OvmfPkg/ResetSystemLib: introduce the DxeResetSystemLib instance

The BaseResetSystemLib instance is not suitable for OS runtime, because
its ResetShutdown() implementation calls PciRead16 (OVMF_HOSTBRIDGE_DID).
On q35, this boils down to a memory-mapped config space access -- but we
never ask the OS to map MMCONFIG for runtime.

There are at least three alternatives to approach this:

(1) Investigate "MdePkg/Library/DxeRuntimePciExpressLib", which offers
    some kind of runtime mapping for MMCONFIG.

(2) Consume PciCf8Lib directly, rather than PciLib, in ResetSystemLib.
    Then we'll read OVMF_HOSTBRIDGE_DID from the config space with IO port
    accesses on q35 too, not just on i440fx. IO ports don't depend on page
    tables.

(3) In the lib constructor, cache "mAcpiPmBaseAddress" based on
    "PcdOvmfHostBridgePciDevId" (which is set by PlatformPei). Then the
    host bridge type will be known at runtime without PCI config space
    accesses.

This patch follows approach (3), in order to mirror AcpiTimerLib.

Notes:

* This patch is best viewed with "git show --find-copies-harder -C43".

* PCDs are not usable in the DXE_CORE, as the PCD PPI is gone, and the PCD
  protocol is not available yet. (The DXE_CORE does consume ResetSystemLib
  in practice, when OVMF is built with -D SOURCE_DEBUG_ENABLE.)

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2675
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200417153751.7110-7-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Rebecca Cran <rebecca@bsdio.com>
[lersek@redhat.com: move third Note (with repro info) to BZ comment]