qemu: remove duplicated code for allocating spice ports
    
    We have two different places that needs to be updated while touching
    code for allocation spice ports.  Add a bool option to
    'qemuProcessSPICEAllocatePorts' function to switch between true and fake
    allocation so we can use this function also in qemu_driver to generate
    native domain definition.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>