char: remove class kind field

char: remove class kind field

The class kind is necessary to lookup the chardev name in
qmp_chardev_add() after calling qemu_chr_new_from_opts() and to set
the appropriate ChardevBackend (mainly to free the right
fields).

qemu_chr_new_from_opts() can be changed to use a non-qmp function
using the chardev class typename. Introduce qemu_chardev_add() to be
called from qemu_chr_new_from_opts() and remove the class chardev kind
field. Set the backend->type in the parse callback (when non-common
fields are added).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>