migration: Fix rare hang of migration_channel_read_peek()

In an unlikely case, when a migration stream is attached to the destination
QEMU and only send <4 bytes to the channel as magic, it's possible that
migration_channel_read_peek() may spin forever.

Fix it by adding a manual sleep for partial read.

Since the path isn't attached to a coroutine, it means when partial read
happens, there's yet not much we can do but hang the main thread, it will
happen even for len==0 case.  It means monitors can hang due to this,
either partial read or no data arrived (but connection established).

Leave this for later, the hope is this is extremely rare in production.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889
Reported-by: Feifan Qian <bea1e@proton.me>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Link: https://lore.kernel.org/r/20260812124327.2572363-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>