migration: Create the postcopy preempt channel asynchronously

migration: Create the postcopy preempt channel asynchronously

This patch allows the postcopy preempt channel to be created
asynchronously.  The benefit is that when the connection is slow, we won't
take the BQL (and potentially block all things like QMP) for a long time
without releasing.

A function postcopy_preempt_wait_channel() is introduced, allowing the
migration thread to be able to wait on the channel creation.  The channel
is always created by the main thread, in which we'll kick a new semaphore
to tell the migration thread that the channel has created.

We'll need to wait for the new channel in two places: (1) when there's a
new postcopy migration that is starting, or (2) when there's a postcopy
migration to resume.

For the start of migration, we don't need to wait for this channel until
when we want to start postcopy, aka, postcopy_start().  We'll fail the
migration if we found that the channel creation failed (which should
probably not happen at all in 99% of the cases, because the main channel is
using the same network topology).

For a postcopy recovery, we'll need to wait in postcopy_pause().  In that
case if the channel creation failed, we can't fail the migration or we'll
crash the VM, instead we keep in PAUSED state, waiting for yet another
recovery.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Manish Mishra <manish.mishra@nutanix.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220707185509.27311-1-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>