event: fix event-handling data race

event: fix event-handling data race

This bug has been present since before the time that commit
f8a519 (Dec 2008) tried to make the dispatch loop re-entrant.

Dereferencing eventLoop.handles outside the lock risks crashing, since
any other thread could have reallocated the array in the meantime.
It's a narrow race window, however, and one that would have most
likely resulted in passing bogus data to the callback rather than
actually causing a segv, which is probably why it has gone undetected
this long.

* daemon/event.c (virEventDispatchHandles): Cache data while
inside the lock, as the array might be reallocated once outside.