Currently, push & pop from event queue (both server & client side)
rely on lock from higher levels, e.g. on driver lock (qemu),
private_data (remote), ...; This alone is not sufficient as not
every function that interacts with this queue can/does lock,
esp. in client where we have a different approach, "passing
the buck".

Therefore we need a separate lock just to protect event queue.

For more info see:
https://bugzilla.redhat.com/show_bug.cgi?id=743817