linux-user: writev Partial Writes

Although not technically not required by POSIX, the writev system call will
typically write out its buffers individually.  That is, if the first buffer
is written successfully, but the second buffer pointer is invalid, then
the first chuck will be written and its size is returned.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
