Depending on what object a file descriptor refers to a different
type of IO channel will be needed - either a QIOChannelFile or
a QIOChannelSocket. Introduce a qio_channel_new_fd() method
which will return the appropriate channel implementation.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>