commit 873ed75 ("misc: sanity check VM config for nested virtualization")
requires that the guest_flag tag can't be empty, or it will fail to build.
This patch changes adl instances of "<guest_flag></guest_flag>" to
"<guest_flag>0</guest_flag>".
Tracked-On: #5923
Signed-off-by: Jiang, Yanting <yanting.jiang@intel.com>