migration: add FEATURE_SEEKABLE to QIOChannelBlock

migration: add FEATURE_SEEKABLE to QIOChannelBlock

Enable the use of the mapped-ram migration feature with savevm/loadvm
snapshots by adding the QIO_CHANNEL_FEATURE_SEEKABLE feature to
QIOChannelBlock. Implement io_preadv and io_pwritev methods to provide
positioned I/O capabilities that don't modify the channel's position
pointer.

Signed-off-by: Marco Cavenati <Marco.Cavenati@eurecom.fr>
Link: https://lore.kernel.org/r/20251010115954.1995298-2-Marco.Cavenati@eurecom.fr
Signed-off-by: Peter Xu <peterx@redhat.com>