When the user creates a LUKS-encrypted qcow2 image using the qemu-img
program, the passphrase is hashed using PBKDF2 with a dynamic
number of iterations. The number of iterations is determined by
measuring thread cpu time usage, such that it takes approximately
2 seconds to compute the hash.

Because Darwin doesn't implement getrusage(RUSAGE_THREAD), we get an
error message:
> qemu-img: test.qcow2: Unable to calculate thread CPU usage on this platform
for this command:
> qemu-img create --object secret,id=key,data=1234 -f qcow2 -o 'encrypt.format=luks,encrypt.key-secret=key' test.qcow2 100M

This patch implements qcrypto_pbkdf2_get_thread_cpu() for Darwin so that
the above command works.

Signed-off-by: Jungmin Park <pjm0616@gmail.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>