Vector: Support copy from host to empty vector

[karlrupp]

It is handy to make possible the following:

std::vector<T> host_x(N);
viennacl::vector<T> x;
viennacl::copy(host_x, x);

in order to not prematurely require a size specification for x. Note that the three lines above may not even be in the same translation unit, so it may be difficult to provide a size for x at the point of its creation.

[karlrupp]

Fixed here: a89695e