OvmfPkg: PciHostBridgeLib: implement PciHostBridgeFreeRootBridges()

This function has no counterpart in OvmfPkg/PciHostBridgeDxe/, but the
PciHostBridgeLib class requires it.

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

OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.c

@@ -215,7 +215,17 @@ PciHostBridgeFreeRootBridges (
   UINTN           Count
   )
 {
-  return;
+  if (Bridges == NULL && Count == 0) {
+    return;
+  }
+  ASSERT (Bridges != NULL && Count > 0);
+
+  do {
+    --Count;
+    UninitRootBridge (&Bridges[Count]);
+  } while (Count > 0);
+
+  FreePool (Bridges);
 }