events: don't hold events lock when dispatching free callbacks
    
    The _event_timeout_remove and _event_handle_remove methods
    were holding onto the global lock when invoking the free
    callback. This is a violation of the libvirt events API
    contract which requires free callbacks to be invoked in
    a re-entrant safe context.
    
    Based on commit dd17c3cc587c73a8c915238f9d9a3e200e89c93f from
    libvirt-glib.
    Original author: Daniel P. Berrange <berrange@redhat.com>
    
    Related to: rhbz#1243228