OvmfPkg: XenHypercallLib: introduce XenHypercallIsAvailable()

Similarly to QemuFwCfgLib, we prefer mellow library construction code and
an explicit "are you available" query function in the XenHypercallLib
class. In this step we introduce that query function, but move no client
code to it yet.

Suggested-by: Jordan Justen <jordan.l.justen@intel.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>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17000 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/Include/Library/XenHypercallLib.h

@@ -16,6 +16,22 @@
 #ifndef __XEN_HYPERCALL_LIB_H__
 #define __XEN_HYPERCALL_LIB_H__
 
+/**
+  Check if the Xen Hypercall library is able to make calls to the Xen
+  hypervisor.
+
+  Client code should call further functions in this library only if, and after,
+  this function returns TRUE.
+
+  @retval TRUE   Hypercalls are available.
+  @retval FALSE  Hypercalls are not available.
+**/
+BOOLEAN
+EFIAPI
+XenHypercallIsAvailable (
+  VOID
+  );
+
 /**
   This function will put the two arguments in the right place (registers) and
   invoke the hypercall identified by HypercallID.