migration/postcopy: Initialize blocktime context only until listen

Before this patch, the blocktime context can be created very early, because
postcopy_ram_supported_by_host() <- migrate_caps_check() can happen during
migration object init.

The trick here is the blocktime context needs system vCPU information,
which seems to be possible to change after that point.  I didn't verify it,
but it doesn't sound right.

Now move it out and initialize the context only when postcopy listen
starts.  That is already during a migration so it should be guaranteed the
vCPU topology can never change on both sides.

While at it, assert that the ctx isn't created instead this time; the old
"if" trick isn't needed when we're sure it will only happen once now.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250613141217.474825-10-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

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