migration: Take BQL slightly longer in postcopy_start()

migration: Take BQL slightly longer in postcopy_start()

This paves way for some follow up patch to modify migration states at the
end of postcopy_start(), which should better be with the BQL so that
there's no way of concurrent cancellation.

So we'll do something slightly more with BQL but they're really trivial,
hopefully nothing will really chance with this.

A side benefit is we can drop another explicit lock() in failure path.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-11-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>