OvmfPkg: Add PCD for Host Bridge dev. ID (PcdOvmfHostBridgePciDevId)

OvmfPkg: Add PCD for Host Bridge dev. ID (PcdOvmfHostBridgePciDevId)

Set from PEI, this PCD allows subsequent stages (specifically
DXE_DRIVER and DXE_RUNTIME_DRIVER) to infer the underlying platform
type (e.g. PIIX4 or Q35/MCH) without the need to further query the
Host Bridge for its Device ID.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16374 6f19259b-4bc3-4df7-8a09-765794883524