migration: move income process out of multifd

migration: move income process out of multifd

Move the call to migration_incoming_process() out of multifd code.  It's
a bit strange that we can migration generic calls in multifd code.
Instead, let multifd_recv_new_channel() return a boolean showing whether
it's ready to continue the incoming migration.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180627132246.5576-3-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>