Fix iovec for the case with invalid elements (Lauro Ramos Venancio).

We must call the writev even if an iovec element is invalid. For
example, if the second element is invalid, the linux process the first
one.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5562 c046a42c-6fe2-441c-8c8c-71466251a162
