OvmfPkg/XenBusDxe: Add support to make Xen Hypercalls.

Change in V4:
- Replace the license by the commonly used file header text.
- add file header to XenHypercall.h (license, copyright, brief desc)

Change in V3:
- adding IA32 support. (not reviewed yet)
  both XenBusDxe/Ia32/hypercall.{S,asm} file are new

Change in V2:
- file header, copyright
- Add License
- Add push/pop instruction.
- fix types
- Comment of exported functions
- Improve coding style
- Add error handling in the main init function (of the drivers)
- Comment assembly

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16260 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/XenBusDxe.c

@@ -29,6 +29,8 @@
 
 #include "XenBusDxe.h"
 
+#include "XenHypercall.h"
+
 
 ///
 /// Driver Binding Protocol instance
@@ -264,6 +266,8 @@ NotifyExitBoot (
   @retval EFI_SUCCESS              The device was started.
   @retval EFI_DEVICE_ERROR         The device could not be started due to a device error.Currently not implemented.
   @retval EFI_OUT_OF_RESOURCES     The request could not be completed due to a lack of resources.
+  @retval EFI_UNSUPPORTED          Something is missing on the system that
+                                   prevent to start the edvice.
   @retval Others                   The driver failded to start the device.
 
 **/
@@ -295,6 +299,20 @@ XenBusDxeDriverBindingStart (
   mMyDevice = Dev;
   EfiReleaseLock (&mMyDeviceLock);
 
+  Status = XenHyperpageInit (Dev);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((EFI_D_ERROR, "XenBus: Unable to retrieve the hyperpage.\n"));
+    Status = EFI_UNSUPPORTED;
+    goto ErrorAllocated;
+  }
+
+  Status = XenGetSharedInfoPage (Dev);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((EFI_D_ERROR, "XenBus: Unable to get the shared info page.\n"));
+    Status = EFI_UNSUPPORTED;
+    goto ErrorAllocated;
+  }
+
   Status = gBS->CreateEvent (EVT_SIGNAL_EXIT_BOOT_SERVICES, TPL_CALLBACK,
                              NotifyExitBoot,
                              (VOID*) Dev,