migration: Move yank outside qemu_start_incoming_migration()

Starting from commit b5eea99e, qmp_migrate_recover() calls unregister
before calling qemu_start_incoming_migration(). I believe it wanted to mitigate
the next call to yank_register_instance(), but I think that's wrong.

Firstly, if during recover, we should keep the yank instance there, not
"quickly removing and adding it back".

Meanwhile, calling qmp_migrate_recover() twice with b5eea99e will directly
crash the dest qemu (right now it can't; but it'll start to work right after
the next patch) because the 1st call of qmp_migrate_recover() will unregister
permanently when the channel failed to establish, then the 2nd call of
qmp_migrate_recover() crashes at yank_unregister_instance().

This patch fixes it by moving yank ops out of qemu_start_incoming_migration()
into qmp_migrate_incoming.  For qmp_migrate_recover(), drop the unregister of
yank instance too since we keep it there during the recovery phase.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210629181356.217312-2-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>