Working AHCI DMA IPC from yellowstone to denali.

We have a weird bug in context switching where a process's rsp0 ends up
pointing at the wrong value sometimes which crashes the OS.

sys/denali/ahci/command.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <mammoth/memory_region.h>
+#include <stdint.h>
+
+#include "ahci/ahci.h"
+
+class Command {
+ public:
+  virtual ~Command();
+  virtual void PopulateFis(uint8_t* command_fis) = 0;
+  virtual void PopulatePrdt(PhysicalRegionDescriptor* prdt) = 0;
+  virtual void Callback() = 0;
+};
+
+class DmaReadCommand : public Command {
+ public:
+  typedef void (*DmaCallback)(uint64_t, uint64_t, uint64_t);
+  DmaReadCommand(uint64_t lba, uint64_t sector_cnt, DmaCallback callback);
+
+  virtual ~DmaReadCommand() override;
+
+  void PopulateFis(uint8_t* command_fis) override;
+  void PopulatePrdt(PhysicalRegionDescriptor* prdt) override;
+
+  void Callback() override;
+
+ private:
+  uint64_t lba_;
+  uint64_t sector_cnt_;
+  DmaCallback callback_;
+  MappedMemoryRegion region_;
+};