crypto: use uint64_t for pbkdf iteration count parameters

The qcrypto_pbkdf_count_iters method uses a 64 bit int
but then checks its value against INT32_MAX before
returning it. This bounds check is premature, because
the calling code may well scale the iteration count
by some value. It is thus better to return a 64-bit
integer and let the caller do range checking.

For consistency the qcrypto_pbkdf method is also changed
to accept a 64bit int, though this is somewhat academic
since nettle is limited to taking an 'int' while gcrypt
is limited to taking a 'long int'.

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