crypto: make PBKDF iterations configurable for LUKS format

As protection against bruteforcing passphrases, the PBKDF
algorithm is tuned by counting the number of iterations
needed to produce 1 second of running time. If the machine
that the image will be used on is much faster than the
machine where the image is created, it can be desirable
to raise the number of iterations. This change adds a new
'iter-time' property that allows the user to choose the
iteration wallclock time.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
