qemu: Don't reset "events" migration capability
    
    When creating v3.2.0-77-g8be3ccd04 commit, I completely forgot that one
    migration capability is very special. It's the "events" capability which
    tells QEMU to report "MIGRATION" events. Since libvirt always wants the
    events, it is enabled in qemuConnectMonitor and the rest of the code
    should not touch it.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1439841
    https://bugzilla.redhat.com/show_bug.cgi?id=1441165
    
    Messed-up-by: Jiri Denemark <jdenemar@redhat.com>
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>