chardev: add support for qapi-based chardev initialization

This patch add support for a new way to initialize chardev devices.
Instead of calling a initialization function with a QemuOpts we will
now create a (qapi) ChardevBackend, optionally call a function to
fill ChardevBackend from QemuOpts, then go create the chardev using
the new qapi code path which is also used by chardev-add.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>