timeout too short when looking for swap partitions in preseed network installs

Hello,

I consistently run into this issue: I use preseeded network install;
the preseed file instructs D-I to use the whole disk and format with
LVM (see attaached "debconf-debug-output.png"). The partitions are
regularly created, but D-I stops complaining that
"/dev/mapper/kickseed_swap_1" is not present (see attached
"preseed-error.png"). If I hit the "Retry" button, everything goes on
just fine.

I think this is a combination of two factors:

- that Ubuntu's D-I does not allow "partman/exception_handler" to be
  preseeded (it's reset to "unseen" state on ever yoccasion).

- that I am installing VMs on a somewhat slow host, so the time it
  takes for a partition to appear in /dev/mapper can be a bit longer
  than usual.