migration: Properly wait on G_IO_IN when peeking messages

migration: Properly wait on G_IO_IN when peeking messages

migration_channel_read_peek() used to do explicit waits of a short period
when peeking message needs retry.  Replace it with explicit polls on the io
channel, exactly like what qemu_fill_buffer() does.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Link: https://lore.kernel.org/r/20251022192612.2737648-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>