virsh: migrate --timeout-postcopy requires --postcopy
virsh: migrate --timeout-postcopy requires --postcopy

Requesting an automated switch to a post-copy migration (using
--timeout-postcopy) without actually enabling post-copy migration (using
--postcopy) doesn't really do anything. Let's make this dependency
explicit to avoid unexpected behavior.

https://bugzilla.redhat.com/show_bug.cgi?id=1455023

Signed-off-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>

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