Otherwise you always get this warning when using --socket-group=users

 vhost socket failed to set group to users (100)

While here, print out the error if chown() fails.

Fixes: f6698f2b ("tools/virtiofsd: add support for --socket-group")
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <162040394890.714971.15502455176528384778.stgit@bahia.lan>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>