iotests: reduce PBKDF iterations when testing LUKS

By default the PBKDF algorithm used with LUKS is tuned
based on the number of iterations to produce 1 second
of running time. This makes running the I/O test with
the LUKS format orders of magnitude slower than with
qcow2/raw formats.

When creating LUKS images, set the iteration time to
a 10ms to reduce the time overhead for LUKS, since
security does not matter in I/O tests.

Previously a full 'check -luks' would take

  $ time ./check -luks
  Passed all 22 tests

  real  23m9.988s
  user  21m46.223s
  sys   0m22.841s

Now it takes

  $ time ./check -luks
  Passed all 22 tests

  real  4m39.235s
  user  3m29.590s
  sys   0m24.234s

Still slow compared to qcow2/raw, but much improved
none the less.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170626123510.20134-4-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
