daemon: Fix core dumps if unix_sock_group is set

daemon: Fix core dumps if unix_sock_group is set

Setting unix_sock_group to something else than default "root" in
/etc/libvirt/libvirtd.conf prevents system libvirtd from dumping core on
crash. This is because we used setgid(unix_sock_group) before binding to
/var/run/libvirt/libvirt-sock* and setgid() back to original group.
However, if a process changes its effective or filesystem group ID, it
will be forbidden from leaving core dumps unless fs.suid_dumpable sysctl
is set to something else then 0 (and it is 0 by default).

Changing socket's group ownership after bind works better. And we can do
so without introducing a race condition since we loosen access rights by
changing the group from root to something else.