OvmfPkg: QemuBootOrder: mark UEFI boot options selected by fw_cfg

OvmfPkg: QemuBootOrder: mark UEFI boot options selected by fw_cfg

This will allow us to identify those UEFI boot options (while keeping
their relative order) that have *not* been selected by fw_cfg.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Michael Chang <mchang@suse.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14667 6f19259b-4bc3-4df7-8a09-765794883524