It's not true that it works for $Z$ small enough.  Consider the $2 \times 2$ case
$$ Z = \pmatrix{t & 2t\cr 2t & 4t\cr} $$
which is positive semidefinite for $t \ge 0$.
Then $$\det(X) = \log(1+t)\log(1+4t) - \log(1+2t)^2 $$
which appears to be negative for all $t > 0$, and certainly is negative for small $t > 0$: its Maclaurin series is
$ \det(X) = -2 t^3 + O(t^4)$.

What is true is that if $Z$ is positive **definite**, $\log(1+tZ)$ will be positive definite for sufficiently small $t > 0$.  This can be obtained using the
Maclaurin series for $\log(1+z)$.