nixos/libvirtd-service: fix for dnsmasq (dir perms 700 -> 755)

Fixes this:

Nov 09 16:18:54 nixos-laptop systemd[1]: Starting Libvirt Virtual Machine Management Daemon...
Nov 09 16:18:54 nixos-laptop dnsmasq[15809]: read /etc/hosts - 2 addresses
Nov 09 16:18:54 nixos-laptop dnsmasq[15809]: failed to load names from /var/lib/libvirt/dnsmasq/default.addnhosts: Permission denied
Nov 09 16:18:54 nixos-laptop dnsmasq[15809]: cannot read /var/lib/libvirt/dnsmasq/default.hostsfile: Permission denied
Nov 09 16:18:55 nixos-laptop systemd[1]: Started Libvirt Virtual Machine Management Daemon.

I don't understand the reason for the original 700 permission bits.
Apparently read-access is needed and Ubuntu also use 755 perms.

Use "chmod" instead of "mkdir -m" to set permissions because mkdir doesn't
modify permissions on existing directories.