OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Fix GCC44 build failure.

Initialize the input parameter FwhInstance in function GetFvbInstance().

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: "Gao, Liming" <liming.gao@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15601 6f19259b-4bc3-4df7-8a09-765794883524