OvmfPkg/LegacyRegion: Support legacy region manipulation of Q35

OvmfPkg/LegacyRegion: Support legacy region manipulation of Q35

Current implementation only supports legacy region of 440 chip.
When QEMU is launched in Q35 mode using CSM enabled OVMF image,
LegacyBios driver fails to start due to the legacy region
[0xC0000, 0xFFFFF] cannot be written.

v2:
 * just updates the comments.

v3:
 * uses PcdOvmfHostBridgePciDevId as Jordan suggested.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Justen Jordan <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>