fix(qemu): update rmmd_attest_get_platform_token()

Update the parameters to rmmd_attest_get_platform_token(), which can now
handle platform tokens larger than 4kB. Since the QEMU sample token is
smaller than 4kB, our implementation remains the same. Take the
opportunity to clean up the function slightly.

Change-Id: Id5a1d576968ebd160d2b79c1f38392d4ecc89421
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>