Relax requirements for "Exec" key when NoDisplay is set.

See https://gitlab.com/qemu-project/qemu/-/issues/440#note_606080304

TLDR: Wayland requires a desktop file to get an icon, but qemu needs a large amount of arguments to run. It is typically forked from a laucher like virt-manager.

There already is an exception for DBUS-activatable applications. It think is is reasonable to add another exception when NoDisplay=true is set.