We are not currently bounding the search to the 1024 bytes
that we allocated, possibly overrunning the buffer.
Use softmmu_strlen_user to find the length and allocate the
correct size from the beginning.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>