zsh-completion: _systemd-nspawn - add more argument completion

zsh-completion: _systemd-nspawn - add more argument completion

filenames will be completed for --image/-i/--bind/--bind-ro/--tmpfs
network interfaces for --network-(interface|macvlan|ipvlan|bridge)
users for --user/-u, yes & no for --register, x86 * x86-64 for
--personality

display a message of the expected argument for --machine/-M/--uuid
--slice/-S/--port/-p/--selinux-*/-Z/-L/--setenv

Allow completing commands(and their options) of the host system for COMMAND