OvmfPkg/OvmfXen: Build platform info HOB in XenPlatformPei
    
    Copy the function BuildPlatformInfoHob() from OvmfPkg/PlatformPei.
    
    QemuFwCfgLib expect this HOB to be present, or fails to do anything.
    InternalQemuFwCfgIsAvailable() from QemuFwCfgPeiLib module will not
    check if the HOB is actually present for example and try to use a NULL
    pointer.
    
    Fixes: cda98df16228 ("OvmfPkg/QemuFwCfgLib: remove mQemuFwCfgSupported + mQemuFwCfgDmaSupported")
    Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
    Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
    Acked-by: Jiewen Yao <jiewen.yao@intel.com>