qemu: properly handle '=' in the VNC socket path
    
    If a domain name contains a '=' and the unix socket path is
    auto-generated or socket path provided by user contains '=' QEMU
    is unable to properly parse the command line.  In order to make it
    work we need to use the new command line syntax for VNC if it's
    available, otherwise we can use the old syntax.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1352529
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>