io: add and implement QIO_CHANNEL_FEATURE_SEEKABLE for channel file

io: add and implement QIO_CHANNEL_FEATURE_SEEKABLE for channel file

Add a generic QIOChannel feature SEEKABLE which would be used by the
qemu_file* apis. For the time being this will be only implemented for
file channels.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240229153017.2221-3-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>