OvmfPkg/QemuKernelLoaderFsDxe: add support for new Linux initrd device path

OvmfPkg/QemuKernelLoaderFsDxe: add support for new Linux initrd device path

Linux v5.7 will introduce a new method to load the initial ramdisk
(initrd) from the loader, using the LoadFile2 protocol installed on a
special vendor GUIDed media device path.

Add support for this to our QEMU command line kernel/initrd loader.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2566
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>