daemon: Always reflect command stderr to stderr when debugging.

daemon: Always reflect command stderr to stderr when debugging.

When debugging (ie. LIBGUESTFS_DEBUG=1 & verbose flag set in daemon)
always reflect any stderr output from commands that we run to
stderr of the daemon, so it is visible.

Previously if stderror == NULL in command*, stderr output was
just eaten and discarded which meant useful error messages could
be lost.