OvmfPkg/OvmfXen: add QemuKernelLoaderFsDxe

OvmfPkg/OvmfXen: add QemuKernelLoaderFsDxe

Without QemuKernelLoaderFsDxe, QemuLoadKernelImage() couldn't download
the kernel, initrd, and kernel command line from QEMU's fw_cfg.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3574
Signed-off-by: Gary Lin <gary.lin@hpe.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>