OvmfPkg: restrict XenPlatformLib to BdsDxe in the IA32, IA32X64, X64 DSCs

OvmfPkg: restrict XenPlatformLib to BdsDxe in the IA32, IA32X64, X64 DSCs

The "OvmfPkg/Library/PlatformBootManagerLib/PlatformBootManagerLib.inf"
library instance is used in the following platform DSC files in edk2:

  OvmfPkg/OvmfPkgIa32.dsc
  OvmfPkg/OvmfPkgIa32X64.dsc
  OvmfPkg/OvmfPkgX64.dsc
  OvmfPkg/OvmfXen.dsc

The Xen customizations are very light-weight in this
PlatformBootManagerLib instance. Isolating them statically, for the sake
of the first three DSC files, would save negligible binary code size, and
would likely worsen code complexity (by way of introducing new internal
interfaces) or blow up source code size (by duplicating almost the entire
lib instance source code). So for now, keep this one bit of Xen dynamism
even on QEMU.

However, because it's only PlatformBootManagerLib now that uses
XenPlatformLib (for the above-stated enlightenment), restrict the
XenPlatformLib class resolution in the first three DSC files to the only
DXE driver that consumes PlatformBootManagerLib (and therefore
XenPlatformLib): BdsDxe. This will cause a build failure later if someone
attempts to call a XenPlatformLib API (that is, tries to re-introduce Xen
enlightenment) in a different module in these non-Xen DSC files.

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>
Message-Id: <20210526201446.12554-44-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>