rwlock: Prevent construction of rwlock_for_* objects

rwlock_for_read, rwlock_for_write are just meant to be interfaces, and
shouldn't be instanciated. We have a private constructor to ensure that
they are never constructed. Unfortunately, because of a gcc (<4.9) bug
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56429) an explicit
"=default" constructor is always public, so this protection doesn't
work at all, and rwlock_for_* objects can be created (and obviously
crash when used).

This patch changes the "default" constructor to an empty constructor,
which does the same thing but avoids this gcc bug.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>