OvmfPkg: XenHypercallLib: introduce XenHypercallIsAvailable()

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