qemu: introduce spiceport chardev backend

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>