qemu: json: Add format strings for optional command arguments
qemu: json: Add format strings for optional command arguments

This patch adds option to specify that a json qemu command argument is
optional without the need to use if's or ternary operators to pass the
list. Additionally all the modifier characters are documented to avoid
user confusion.

agx/libvirt,nertpinx/libvirt,elmarco/libvirt,jardasgit/libvirt,zippy2/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,jfehlig/libvirt,eskultety/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,datto/libvirt,jardasgit/libvirt,crobinso/libvirt,eskultety/libvirt,eskultety/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,taget/libvirt,shugaoye/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,rlaager/libvirt,rlaager/libvirt,eskultety/libvirt,crobinso/libvirt,crobinso/libvirt,libvirt/libvirt,taget/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,datto/libvirt,elmarco/libvirt,libvirt/libvirt,shugaoye/libvirt,elmarco/libvirt,VenkatDatta/libvirt,datto/libvirt,fabianfreyer/libvirt,zippy2/libvirt,datto/libvirt,zippy2/libvirt,andreabolognani/libvirt,nertpinx/libvirt,agx/libvirt,jardasgit/libvirt,taget/libvirt,datto/libvirt,nertpinx/libvirt,rlaager/libvirt,olafhering/libvirt,agx/libvirt,jardasgit/libvirt,rlaager/libvirt,elmarco/libvirt,andreabolognani/libvirt,eskultety/libvirt,olafhering/libvirt,libvirt/libvirt,rlaager/libvirt,andreabolognani/libvirt,taget/libvirt,agx/libvirt,crobinso/libvirt,shugaoye/libvirt,olafhering/libvirt,agx/libvirt,jfehlig/libvirt,libvirt/libvirt,andreabolognani/libvirt,elmarco/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,taget/libvirt,andreabolognani/libvirt,shugaoye/libvirt,olafhering/libvirt,nertpinx/libvirt