add IO requrest 'req_buf' check before reference

This address maybe invalid if a hostile address was set
in hypercall 'HC_SET_IOREQ_BUFFER'.it should be validated
before using.

Update:
  -- save HVA to guest OS's request buffer in hyperviosr
  -- change type of 'req_buf' from 'uint64_t' to 'void *'
  -- remove HPA to HVA translation code when using this addr.
  -- use error number instead of -1 when return error cases.

Signed-off-by: Yonghua Huang <yonghua.huang@intel.com>

hypervisor/include/arch/x86/guest/vm.h

@@ -76,8 +76,8 @@ struct vm_sw_info {
 	struct sw_kernel_info kernel_info;
 	/* Additional information specific to Linux guests */
 	struct sw_linux linux_info;
-	/* GPA Address of guest OS's request buffer */
-	uint64_t req_buf;
+	/* HVA to guest OS's request buffer */
+	void *req_buf;
 };
 
 struct vm_pm_info {