qemu_process: mark auto-generated spice ports as reserved
    
    In case you will specify graphics like this:
    
    <graphics type='spice' port='-1'/>
    
    or
    
    <graphics type='spice' port='-1' tlsPort='6000'/>
    
    libvirt will automatically add autoport='no'.  This leads to an issue
    that in qemuProcessStop() we don't release that port because we are
    releasing both port if autoport=yes or only port marked as reserved.
    
    If autoport=no but we request to generate port via '-1' we need to mark
    that port as reserved in order to release it.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1299696
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>