OvmfPkg: Factor out platform detection (q35 vs. piix4)

OvmfPkg: Factor out platform detection (q35 vs. piix4)

Introduce macros to detect the underlying platform and access its
ACPI power management registers, based on querying the host bridge
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@16372 6f19259b-4bc3-4df7-8a09-765794883524