hyperstart/mock: Don't directly print to stderr
hyperstart/mock: Don't directly print to stderr

Instead of manually printing to stderr, use the built-in testing.Log(f)
functions to output debug messages when the test running is set up to be
verbose.

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

wcwxyz/virtcontainers,containers/virtcontainers,wcwxyz/virtcontainers,containers/virtcontainers,wcwxyz/virtcontainers