libxl: Add optimisation to ev_lock

libxl: Add optimisation to ev_lock

It will often be the case that the lock is free to grab. So we first
try to grab it before we have to fork. Even though in this case the
locks are grabbed in the wrong order in the lock hierarchy (ev_lock
should be outside of CTX_LOCK), it is fine to try without blocking. If
that failed, we will release CTX_LOCK and try to grab both lock again
in the right order.

That optimisation is only enabled in releases (debug=n) so the more
complicated code with fork is actually exercised.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>