io: Introduce a qio_channel_set_feature() helper

Testing QIOChannel feature support can be done with a helper called
qio_channel_has_feature(). Setting feature support, however, was
done manually with a logical OR. This patch introduces a new helper
called qio_channel_set_feature() and makes use of it where applicable.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>