OvmfPkg/XenPlatformPei: Grab RSDP from PVH guest start of day struct

Check if there's a start of the day struct provided to PVH guest, save
the ACPI RSDP address for later.

This patch import import arch-x86/hvm/start_info.h from xen.git.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-13-anthony.perard@citrix.com>

OvmfPkg/Include/Guid/XenInfo.h

@@ -25,6 +25,10 @@ typedef struct {
   /// Hypervisor minor version.
   ///
   UINT16 VersionMinor;
+  ///
+  /// Pointer to the RSDP found in the hvm_start_info provided to a PVH guest
+  ///
+  VOID *RsdpPvh;
 } EFI_XEN_INFO;
 
 extern EFI_GUID gEfiXenInfoGuid;