events: allow to remove disabled timers and handles
    
    Trying to remove a disabled timer or handle will cause
    virt_viewer_events_remove_{handle,timeout}() to return an error
    rather than removing it.
    
    Based on commit 79699d73e6e1b7218e3bd8349d176752f86128b9 from
    libvirt-glib.
    Original author: Christophe Fergeau <cfergeau@redhat.com>
    
    Related to: rhbz#1243228