Make the '--socket-group=' option fail if the group name is unknown:

./tools/virtiofsd/virtiofsd .... --socket-group=zaphod
vhost socket: unable to find group 'zaphod'

Reported-by: Xiaoling Gao <xiagao@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20211014122554.34599-1-dgilbert@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>