qemu: introduce spiceport chardev backend
    
    Add a new backend for any character device.  This backend uses channel
    in spice connection.  This channel is similar to spicevmc, but
    all-purpose in contrast to spicevmc.
    
    Apart from spicevmc, spiceport-backed chardev will not be formatted
    into the command-line if there is no spice to use (with test for that
    as well).  For this I moved the def->graphics counting to the start
    of the function so its results can be used in rest of the code even in
    the future.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>