cutils: replace strdup with g_strdup

cutils: replace strdup with g_strdup

Memory returned by get_relocated_path must be freed with
free or g_free depending on the path that the function
took; Coverity takes exception to this practice.  The
fix lets caller use g_free as is standard in QEMU.

While at it, mention the requirements on the caller in
the doc comment.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>