OvmfPkg: add GUID for the QEMU kernel loader fs media device path

OvmfPkg: add GUID for the QEMU kernel loader fs media device path

In an upcoming patch, we will introduce a separate DXE driver that
exposes the virtual SimpleFileSystem implementation that carries the
kernel and initrd passed via the QEMU command line, and a separate
library that consumes it, to be incorporated into the boot manager.

Since the GUID used for the SimpleFileSystem implementation's device
path will no longer be for internal use only, create a well defined
GUID to identify the media device path.

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>